Gyro Wheel

Thursday September 10, 2009

gyro1Gyroscopes defy human intuition, common sense, and even appear to defy gravity. You’ll find them in aircraft navigation instruments, games of Ultimate Frisbee, fast bicycles, street motorcycles, toy yo-yos, and the Hubble Space Telescope. And of course, the toy gyroscope (as shown here). Gyroscopes are used at the university level to demonstrate the principles of angular momentum, which is what we’re going to learn about here.


If you happen to have one of these toy gyroscopes, pull it out and play with it (although it’s not essential to this experiment). Notice that you can do all sorts of things with it when you spin it up, such as balance it on one finger (or even on a tight string). Wrap one end with string and hold the string vertically and you’ll find the gyro slowly rotates about the vertical string instead of flopping downward (as most objects do in Earth’s gravitational field). But why? Here’s the answer in plain English:


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Imagine a spinning bicycle wheel hanging from a rope. Take a freeze-frame image of the wheel in your mind and make the top part of the wheel is at 12 o’clock, the left side at 9 o’clock, the bottom is 6 o’clock, and the center axle pointing toward you. And the wheel was rotating clockwise. Got it?


The 6 o’clock position wants to move to the left. When the 6 o’clock position gets to the 9 o’clock, it still wants to move left.


The original 9 o’clock position wants to move up. When the 9 o’clock position gets to 12 o’clock, it still wants to move up.


The 12 o’clock position wants to move to the right. When the 12 o’clock position moves to the 3 o’clock, it still wants to move right. See the pattern?


Okay, here’s what you want to see now: as the top and bottom (12 and 6 o’clock) positions of the wheel rotate, the forces cancel each other out. Same with the 3 and 9 o’clock positions. And because the wheel is symmetrical, this occurs for every spot on the wheel. When this happens, the bicycle wheel turns (precesses), instead of falling. This is also why a spinning gyroscope will appear to float at the end of the string instead of dangling.


One more piece to the puzzle: the wheel itself is accelerating. Any object that swings in a circle is accelerating, because in order to move in a circle, you need to be constantly changing your direction (or else you’d be off in a straight line tangent to your circular path). When you swing a bag of oranges around your head, or a yo-yo on a string, or even taking a turn in a car, acceleration is happening. We’ll talk more about that when we do our g-force experiments. For right now, let’s get our hands on a super-cool experiment that you usually only see at science museums or inside physics classrooms.


You’ll need to find:


  • a bicycle wheel (it needs to be detached from a bicycle – note that the front wheel is pretty easy to detach)
  • a piece of rope (about 2-3′)
  • and an office chair

1. Carefully hold the bicycle wheel by the axle and give it a spin. Try to get it to spin as fast as you can but be VERY careful to hold onto it tightly and don’t get your fingers in the spokes. It works well if someone else can spin it for you.


2. While it’s spinning try to move it around. You’ll find that the wheel does not want to be moved around and tries to do its own thing when you move it.


3. Take a string and loop it around one side of the axle of the wheel. Spin the wheel fast and let go of the wheel while holding onto the string. The wheel will stay spinning and defy gravity by staying straight up and down. Really! Try it, it’s very cool!



Download Student Worksheet & Exercises


This experiment demonstrates the power of Newton’s Second Law: Force equals mass times acceleration. The mass is the mass of the wheel, in particular, the mass of the tire. The acceleration is the spinning wheel. Remember, acceleration is not just change in speed but also change in direction. Every point on the spinning wheel is constantly changing direction so the wheel is accelerating.


Since acceleration times mass has to equal force, (math says so) the spinning wheel has a force. This force is strong enough to defy gravity and is what you feel when you hold the axles of the wheel and try to move it. This same force is what keeps a top spinning, why footballs are thrown with a spiral, why satellites spin and so on. It’s pretty incredible to think that a force can be created by nothing more than accelerating mass.


Best Physics Joke

A friend once told me of a funny April Fool’s joke played at his office. He worked at a company that fixed aircraft instruments, where they received all sorts of old airplane parts. One day, his office anonymously received an old instrument from a WWII bomber, and the gyro in this thing was HUGE. Then he had an idea – he removed the gyro from the outer instrument casing, fixed it, spun it up (and it would continue spin for hours), and packed it up in his boss’s briefcase. When his boss went to lunch and picked up his briefcase, can you imagine what happened? (Tell me in the comment field below!)


Exercises


  1. What did it feel like when you tried to turn the wheel after it was spun?
  2. What direction (orientation) does the wheel want to be in?
  3. When you were on the spinning chair/platform, which way did you turn?
  4. If you turned the wheel left, you should have spun the same way, where is the force coming from the pushed you in that direction?
  5. What happened to the wheel while you held on to the string? Did it stay upright, or dangle?
  6. Why do you think it stayed upright?

For Advanced Students:

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When you grab hold of the axle of a spinning bicycle wheel, you feel a ‘push’ in an odd direction. This ‘push’ is called precession and is a wobble from the spin axis because you tried to move it in a direction it doesn’t want to go.


Precession happens when you grab a spinning object at its rotating axis (like the ends of the gyroscope or the axle of a bicycle wheel) and try to move it about. You’ll find a fierce resistance crop up. That’s precession. The question is why? Why does the gyro act like this?


Normally, this area of scientific study is enough to make most 3rd year engineering students cry. The engineering required to model this system are complex, let alone finding the solution to the mathematical differential equations that make up the model itself. So we’re not going to get into the nit-picky stuff, but instead talk about how it actually works using plain English. Here’s what’s going on:


Most gyroscopes are designed to have most of the rotating mass far away from the center axis (think of a thin disk with a heavy rim, like a bike wheel) so it can resist motion in certain directions. A spinning bike wheel stores large amounts of energy. Remember Newton’s first law of motion? (An object in motion tends to stay in motion unless something else interferes.) Any time you try to torque the bike axle, the wheel will try to ‘compensate’ for this and push in a different direction. (Parts of a car engine will also do this – think of the fast-spinning shaft or fan when you try to take a sharp turn.)


Who first thought up this stuff?

German scientist Johann Bohnenberger built the first gyroscope as a giant spinning ball near the start of the 1800s. About two decades later, Walter R. Johnson (an American) shifted the spinning solid sphere into a spinning disc, which was later upgraded to describing the Earth’s rotation by a French mathematician Pierre-Simon Laplace. Léon Foucault attempted to use the gyroscope in experiments that could detect the earth’s rotation (while still being on the planet itself), but his work lacked a frictionless mount (which was later developed), and now the famous Foucault Pendulum is in many science museums across the country. (It’s the one with the 3-story tall, 300-pound brass plumb-bob that knocks over dominoes every 6-14 minutes.)


The first industrial application for the gyroscope was for the military (marine applications, then quickly followed for aircraft) in the early 1900s, followed shortly after by the toy industry. Today gyroscopes are used mostly in navigational systems and inertial guidance systems for ballistic missiles.


You can think of the Earth as a gyro as well. Of precession is the ‘wobble’ a spinning gyro makes when a force is applied (the force in this case is the pull mostly from the sun), then the Earth ‘wobbles’ through one precession cycle about every 26,000 years. What does this mean? It means that the axial poles of the earth are scribing small, slow circles over time (the ‘wobble’). It also means that star positions will slowly change on our gridmarked area of the sky, so every so often we’ll need to update our coordinate systems so we can accurately locate the positions of the stars. (But it’s only a shift of about 1 degree very 70+ years.)


The interesting thing is that the Earth’s precession was actually discovered ages ago by the ancient Greek astronomer Hipparchus (150 B.C.), but it couldn’t be mathematically described until we had Newtonian physics to guide the way (and even then we had a few issues). For example, we had to figure out that the Earth is really not a sphere but more of an ‘squashed sphere’ (imagine squashing a ball of clay slightly between your fingers so that the middle bulges out). And both the Sun and the Moon pull on the bulge (lunisolar precession), which adds more to the ‘wobble’ of the Earth. And did I mention the Sun is not a perfect sphere, either? (It’s actually kind of flat… so that had to be accounted for as well.)


All of this can be mathematically modeled in the world of engineering through a conservation law called Angular Momentum. For those of you who really want to learn more about angular momentum (which is purely your choice, but it is out of the scope of this program because it’s a college-level topic), click here to download a chapter from an advanced textbook.


Advanced students: Download your Gyro Wheel Lab here.


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Downhill Race

Thursday September 10, 2009

Newton’s Second Law is one of the toughest of the laws to understand but it is very powerful. In its mathematical form, it is so simple, it’s elegant. Mathematically it is F=ma or Force = Mass x Acceleration. An easy way to remember that is to think of your mother trying to get you out of bed in the morning. Force equals MA’s coming to get you! (I did mention how bad physics jokes are, right?)


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Newton’s Second Law

In English, Newton’s Second Law can be stated a few different ways:


The more mass something has and/or the faster it’s accelerating, the more force it will put on whatever it hits. F=ma For example, a car colliding at 30 mph will hit a lot harder then a fly colliding at 30 mph.


The more mass something has, the more force that’s needed to get it to accelerate. a=F/m This, by the way, is a mathematical definition for acceleration. For example, it is a lot harder to get a train to accelerate than it is to get a ping pong ball to accelerate.


So force = mass X acceleration. Let’s try a couple of things and see if we can make that make sense.


Now, time to do an experiment. You will need:


  • something slightly slanted, like a slanted driveway or a long board (or even a table propped up on one end)
  • something to move down the slant, like a toy car or ball
  • a stopwatch
  • pen and paper

I’m going to assume you’re using the toy car and a driveway. Feel free to modify the experiment for whatever you are using.


1. Take the toy car to the top of the driveway.


2. Let it go.


3. Watch it carefully as it rolls.


4. If you’d like, you can time the car and mark how far it goes every second like we did in this acceleration experiment.


5. If you time it, measure the distances it went each second and write them down.



 
Download Student Worksheet & Exercises


What I’m hoping you will see here is that the car accelerates from zero to a certain velocity but then stays at that velocity as it continues down the driveway. In other words, it reaches its terminal velocity. If you timed and marked the distances you should see that the car goes the same distance each second if it is indeed staying at a constant velocity. If the object you are using to roll down the slant, continues to accelerate down the entire ramp, see if you can find something that has more friction to it (a toy car that doesn’t roll quite to easily, for example).


Ok, so what’s going on? F=ma right? Acceleration can’t happen without force. What two forces are effecting the car? (Imagine the Jeopardy theme song here). If you said gravity and friction, give yourself a handshake. When the car is going at a constant velocity, is it accelerating? Nope, acceleration is a change in speed or direction.


“But you just said two forces are effecting my little car and that force causes acceleration and yet my car is not accelerating. Why not?”


Well, there’s one little thing I haven’t mentioned yet, which is why we did this experiment. In this case, the force of gravity pulling on the car and the force of friction pushing on the car is equal (remember, that’s terminal velocity right?). So the net force on the car is zero. The pulling force is equal to the pushing force so there is zero force on the car. Force is measured in Newton’s (name sounds familiar right?) so imagine that there are 3 Newtons of force pulling on the car due to gravity and 3 Newtons of force pushing on the car due to friction. 3 – 3 = 0. Zero force equals zero acceleration because you need force to have acceleration. By the way, 1 Newton is about the same amount of force that it takes to lift a full glass of milk.


Advanced Students: Download your Downhill Races Lab here.


Exercises 


  1. You should notice a difference between these graphs and the ones from the driveway races. What is it? (Hint: look to second half of the graph.)
  2.  The first graph doesn’t continue to curve, but straightens out.  What does this mean about the velocity?
  3.  in the second graph, the slope flattens out completely, what does this mean about the acceleration?
  4.  If the acceleration is zero, what does that mean about the net force?
  5.  What are the forces acting on the toy car as it is going down the ramp?
  6.  Name 3 other examples

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TaaDaaaaa!!!

Thursday September 10, 2009

Ever wonder how magicians work their magic? This experiment is worthy of the stage with a little bit of practice on your end.


Here’s how this activity is laid out: First, watch the video below. Next, try it on your own. Make sure to send us your photos of your inventions here!


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For this incredibly easy, super-amazing experiment, you’ll need to find:


  • a plastic cup
  • hard covered book
  • toilet paper tube
  • a ball that’s a bit smaller then the opening of the cup but larger than the opening of the toilet paper tube (you can also use an egg when you really get good at this trick!)

1. Put the cup on a table.


2. Put the book on top of the cup.


3. This is the tricky part. Put the toilet paper tube upright on the book, exactly over the cup.


4. Now put the ball on top of the toilet paper tube.


5. Check again to make sure the tube and the ball are exactly over the top of the cup.


6. Now, hit the book on the side so that it moves parallel to the table. You want the book to slide quickly between the cup and the tube.


7. If it works right, the book and the tube fly in the direction you hit the book. The ball however falls straight down and into the cup.


8. If it works say TAAA DAAA!



 
Download Student Worksheet & Exercises


This experiment is all about inertia. The force of your hand got the book moving. The friction between the book and the tube (since the tube is light it has little inertia and moves easily) causes the tube to move. The ball, which has a decent amount of weight, and as such a decent amount of inertia, is not effected much by the moving tube. The ball, thanks to gravity, falls straight down and, hopefully, into the cup. Remember the old magician’s trick of pulling the table cloth and leaving everything on the table? Now you know how it’s done. “Abra Inertia”!


So inertia is how hard it is to get an object to change its motion, and Newton’s First Law basically states that things don’t want to change their motion. Get the connection?


Exercises 


  1. What are two different pairs of forces in this experiment?
  2. Explain where Newton’s Three Laws of motion are observed in this experiment.

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Rocket Car

Thursday September 10, 2009
busLet's take a good look at Newton's Laws in motion while making something that flies off in both directions. This experiment will pop a cork out of a bottle and make the cork fly go 20 to 30 feet, while the vehicle moves in the other direction!

This is an outdoor experiment. Be careful with this, as the cork comes out with a good amount of force. (Don’t point it at anyone or anything, even yourself!)

Here's what you need to find:

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  • toy car
  • baking soda and vinegar (OR alka-seltzer and water)
  • tape
  • container with a tight-fitting lid (I don't recommend glass containers... see if you can find a plastic one like a film canister or a mini-M&M container.)


There are two ways to do this experiment. You can either strap the bottle to the top of a toy car and use baking soda and vinegar, OR use effervescent tablets (like generic brands of alka seltzer) with this modified pop rocket (which you can strap to a toy car, or add wheels to the film canister itself by poking wooden skewers through milk jug lids for wheels and sliding the skewer through a straw to make the axle). Both work great, and you can even do both! This is an excellent demonstration in Newton's Third Law, inertia, and how stuff works differently here than in outer space. Here's what you do:

1. Strap the bottle to the top of the toy car or bus with the duct tape. You want the opening of the bottle to be at the back of the vehicle.

2. Put about one inch of vinegar into the bottle.

3. Shove a wad of paper towel as far into the neck of the bottle as you can. Make sure the wad is not too tight. It needs to stick into the neck of the bottle but not too tightly.

4. Pour baking soda into the neck of the bottle. Fill the bottle from the wad of paper all the way to the top of the bottle.

5. Now put the cork into the bottle fairly tightly. (Make sure the corkscrew didn't go all the way through the cork, or you'll have leakage issues.)

6. Now tap the whole contraption hard on the ground outside to force the wad of paper and the baking soda into the bottle.

7. Give the bottle a bit of a shake.

8. Set it down and watch. Do not stand behind the bus where the cork will shoot.

9. In 20 seconds or less, the cork should come popping off of the bottle.

What you should see is the cork firing off the bottle and going some 10 or 20 feet. The vehicle should also move forward a foot or two. This is Newton’s Third law in action. One force fired the cork in one direction. Another force, equal and opposite, moved the car in the other direction. Why did the car not go as far as the cork? The main reason is the car is far heavier then the cork. F=ma. The same force could accelerate the light cork a lot more than the heavier car.

Download Student Worksheet [/am4show]

Why bother with seat belts and helmets?

Thursday September 10, 2009
Every wonder why you have to wear a seat belt or wear a helmet? Let's find out (safely) by experimenting with a ball.

You will need to find:
  • a car
  • a licensed driver
  • a ball
[am4show have='p8;p9;p12;p39;p92;' guest_error='Guest error message' user_error='User error message' ] 1. Next time you go for a ride bring a tennis ball with you in the car.

2. Sit in the back seat and put the tennis ball in the seat next to you.

3. Now watch the ball carefully as the car moves. See how it moves around the seat? (Try not to let it get on the floor and roll around. It might roll under the pedals and that would be bad.)

4. Pretty easy huh? As the car moves forward at 20 mph, everything in the car is moving forward at 20 mph. Everything in the car has the same inertia. If the car were to stop suddenly, everything in the car that’s not bolted down, still moves forward at 20 mph until it hits something. An object at rest tends to stay at rest, an object in motion tends to stay in motion. Right? So, if the car stops quickly, the tennis ball continues to move forward until it hits something. If the car turns, the ball continues to go in the direction it was going a second ago, so it rolls around the seat. What would happen if the car stopped suddenly and you weren’t wearing a seat belt? Yup, you’d fly forward at whatever speed the car was going until you crashed into something in the car. See now why seat belts are your best friend?

Download Student Worksheet [/am4show]

Newton’s Wagon

Thursday September 10, 2009

This is a quick and easy demonstration of how to teach Newton’s laws with minimal fuss and materials. All you need is a wagon, a rock, and some friends. We’re going to do a few totally different experiments using the same materials, though, so keep up with the changes as you read through the experiment.


Remember that Newton covers a few different ideas. First, there’s the idea that objects in motion will stay going they way they’re headed, unless something gets in the way. Then there’s the resistance to motion (objects at rest tend to stay put), as well as force being proportional to how fast you can get something to move (acceleration). And lastly, there’s the idea that forces happen in pairs – if you shoot something one direction, you’re going to feel a kick in the opposite direction. Ready to see these ideas in action? Let’s go…


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Here’s what you need:


  • wagon
  • a rock
  • friends

Part 1: Newton’s First Law (& Inertia)

Let’s really figure out what this ‘inertia’ thing from Newton’s first law is all about using the wagon and friends. Here’s what you need to do:


1. Pull the wagon down the sidewalk.


2. Try to stop as quickly as you can. Be careful. You could get run over by the wagon if you’re not careful.


3. Put a friend in the wagon and repeat steps 1 and 2.


4. Put another friend in the wagon and repeat steps 1 and 2.


5. Again…pretty easy huh?!


You may have noticed that the more friends (the more weight) you had in the wagon the harder it was to get moving and the harder it was to stop. This is inertia. The more weight something has the more inertia it has and the harder it is to get it to go and to stop!


Quiz question: Will a lighter or heaver race car with the same engine win a short-distance race (like the quarter-mile)? Tell me in the comments below what you think!


Advanced students: Download your First Law Lab here. (This is the first of TWO advanced labs!)


Download Student Worksheet & Exercises


Part 2: Newton’s Second Law

Now we’re going to experiment with Newton’s Second Law that deals with force, mass, and acceleration. Are you ready?


1. Start with an empty wagon…


2. Pull it and try to get it to go as fast as it can, as fast as you can. In other words, get it to accelerate.


3. Now add weight. Put something in the wagon that weighs at least 50 lbs. or so (a nice, solid kid comes to mind)


4. Pull it again and get it to go as fast as it can as fast as you can.


5. Add more weight and do it again.


6. Keep adding weight until you have a very difficult time getting it to accelerate.


So what happened here? Force equals mass x acceleration. The mass was the wagon. The force was you pulling. The acceleration was how fast you could get it to speed up. The heavier you got the wagon (the more mass there was) the harder (the more force) you had to pull to get the wagon to move (to accelerate).


Part 3: Newton’s Third Law

Now let’s work with Newton’s Third Law: for every action, there is an equal and opposite reaction. If this next experiment doesn’t work don’t worry about it. You need a fairly low friction skateboard or wagon to make this work, so that’s why it’s here last. You need: A skateboard or a wagon, the heaviest thing you can throw safely, and a sidewalk.


1. Sit in the wagon or on the skateboard (please do not stand up).


2. Throw the heavy thing as hard as you can. Please be careful not to hit anybody or anything.


At this point, you should know what should happen, so what do you think? If you said that the throw forward would move you backward, you’re right! Next time you’re in a small canoe, toss the rock and see what happens to you and your boat. (Any guesses?)


Advanced students: Download your Newton’s Wagon Advanced Lab here (This is the second of TWO advanced labs!)


In this next experiment, we are going to combine the concepts of Newton’s Second Law, acceleration and terminal velocity to explore Newton’s Second a little further. Are you ready?


Exercises 


  1. What is inertia?
  2. What is Newton’s First Law?
  3. Will a lighter or heavier race car with the same engine win a short-distance race (like the quarter-mile)?
  4. What concept does Newton’s Second Law of Motion deal with?
  5. What is momentum?
  6. What is the equation for Newton’s Second Law?
  7. What is Newton’s Third law?
  8. Give three examples of forces in pairs.
  9. A rope is attached to a wall. You pick up the rope and pull with all you’ve got. A scientist walks by and adds a force meter to the rope and measures you’re pulling with 50 Newtons. How much force does the wall experience?
  10. Can rockets travel in space if there’s nothing to push off of? Explain your answer.

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Look Out Below!

Thursday September 10, 2009

If you jump out of an airplane, how fast would you fall? What’s the greatest speed you would reach? Let’s practice figuring it out without jumping out of a plane.


This experiment will help you get the concept of velocity by allowing you to measure the rate of fall of several objects. It’s also a great experiment to record in your science journal.


First, you’ll need to find your materials:


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  • stop watch
  • feathers (or small pieces of paper, plastic bag or anything light and fluffy)
  • a tape measure
  • If you’re crunching numbers, you’ll also need a calculator.

Now here’s how to do the experiment:


1. Get 5 or so different light and fluffy objects. Feathers of different size, small strips of paper, parts of a plastic bag, cotton balls, whatever is handy.


2. Make a prediction by writing down the objects you chose in order of how fast you think they will fall. The fastest on top, the slowest on the bottom. Leave space to the right of your prediction so that you can write in your conclusions and then compare the two.


3. Make a table with two columns. Use one column to fill in the name of the items. Use the second column to write down the time it took each object to fall.


4. Drop the different items and time them from the moment you let go to the moment they hit the ground. Be sure to drop each item from about the same height. The higher the better. Just be sure not to fall off anything! We don’t want to measure your velocity!! You might want to drop them two or three times to get an average time.


5. Now compare the items. Which one fell the least amount of time (dropped the fastest)? Which one fell the most amount of time (dropped the slowest)? Write your results next to your hypothesis. By the way, did you find anything that dropped slower than a feather? I have seen very few things that take longer to fall straight down than a feather.



 
Download Student Worksheet & Exercises


Did you see how many of your objects stopped accelerating very quickly? In other words, they reached their terminal velocity soon after you let them go and they fell all the way to the ground at that same constant velocity. This is why a parachute is a sky-diver’s best friend! A human has a decent amount of air resistance but he or she can reach a lethal dose of velocity (120 mph) if dropped from a great height. The parachute increases the air resistance so that the terminal velocity of that sky-diver is quite a bit safer!


Exercises


  1. What is velocity?
  2. How do acceleration and deceleration relate to velocity?
  3. How do we know when an object has reached terminal velocity?

Taking it Further for advanced students:

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We can do a little math here and figure out the actual velocity of your objects. Here’s how to do it:


Measure the height from which you dropped each object. Now take the height of the drop and divide that by the number of seconds it took for it to drop that distance. That’s the velocity of that object. For example, My “from-under-the-couch-six-month-old-dust-bunny” took 3 seconds to fall 6 feet. I take 6 feet and divide it by 3 seconds to get 2 feet/second.


The velocity of my dust bunny is 2 feet/second downward.


Remember, that velocity has a directional component as well as a number. Add a little more math, and I can predict how long my dust bunny will take to fall 15 feet. Take the distance (15 feet) and divide it by the velocity (2 feet/second) and I get 7.5 seconds. It will take my dust bunny 7.5 seconds to fall 15 feet. Hmmm, maybe we should call it a dust snail.


Have you noticed something here? In this experiment, we used a different formula to find out how far something would fall over a given time.


What’s going on? In Unit 1, we ignored their terminal velocity. Those things were in free fall and accelerating (gaining velocity) all the way to the ground. They were never going the same velocity for the entire trip. So, we needed to use the gravitational constant 32 ft/s² in the equation d=1/2 gt² to determine how far something fell in a given amount of time.


For this unit, we are dealing with things that are at an almost constant velocity, (since they reach their terminal velocity quickly) so we can use the much simpler equation d=vt (d is distance, v is velocity and t is time). In the problems we’ve done in this lesson plan, we have modified that formula to find how long the fall took so we’ve used t=d/v.


If you’d like to solve for v you would use v=d/t. Isn’t algebra fun?


Advanced students: Download your terminal velocity lab here.


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G-Force

Thursday September 10, 2009

A common misconception in science is that centrifugal and centripetal force (or acceleration) are the same thing. These two terms constantly throw students into frenzy, mostly because there is no clear definition in most textbooks. Here’s the scoop: centripetal and centrifugal force are NOT the same thing!


This experiment is mostly for Advanced Students, but here’s a quick lesson you can do with your younger students…


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Before we jump in, let’s recap what we’ve learned so far. A ball sitting still has a position you can chart on a map (latitude, longitude, and altitude), but no velocity or acceleration, because it’s not moving. When you kick the ball, that’s when it gets interesting. The second your toe touches the ball, things start to change. Velocity is a change of position. If you kick the ball ten feet, and it takes five seconds to go the distance, the average speed of the ball is 2 feet per second (about 1.4 MPH).


The trickier part of this scenario has to do with acceleration, which is the change in velocity. When you drive on the freeway at a constant 65 MPH, your acceleration is zero. Your speed does not change, so you have no acceleration. Your position is constantly changing, but you have constant speed. For example, when you enter the freeway, your speed changes from zero to 65 MPH in, say, ten seconds. Your acceleration is greatest when your foot first hits the gas – when your speed changes the most – when you’re moving from zero to a higher speed.


There’s an interesting effect that happens when you travel in a curve. You can feel the effect of a different type of acceleration when you suddenly turn your car to the right – you will feel a push to the left. If you are going fast enough and you take the turn hard enough, you can get slammed against the door. So – who pushed you?


Think back to the first law of motion. An object in motion tends to stay in motion unless acted upon by an external force. This is the amazing part – the car is the external force. Your body was the object in motion, wanting to stay in motion in a straight line. The car turns, and your body still tries to maintain its straight path, but the car itself gets in the way. When you slam into the car door, the car is turning itself into your path, forcing you to change direction.


This effect is true when you travel in a car or in a roller coaster. It’s the reason the water stays in the bucket when you swing it over your head. Physical motion is everywhere, challenging toddlers learning to walk as well as Olympic downhill skiers to go the distance. Here’s a quick experiment you can do right now to wrap your head around this idea:


Here’s what you need to find for these experiments:


  • bucket
  • water
  • outdoor area
  • you
  • clear tubing (about 12-18″ long)
  • nylon or metal barbed union that fits inside the tubing
  • empty soda bottle
  • clean wine cork
  • string

Bucket Splash

Fill a bucket half-full with water. Grasp the handle and swing it over your head in a circle in the vertical direction. Try spinning around while holding the handle out in front of your chest to swing it in the horizontal plane. Vary your spin speed to find the minimum!


Now let’s take a deeper look at centripetal, centrifugal, and how you can measure the g-force when taking a sharp turn in your car:


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For Advanced Students:

Centripetal (translation = “center-seeking” ) is the force needed to keep an object following a curved path. Remember how objects will travel in a straight line unless they bump into something or have another force acting on it (gravity, drag force, etc.)? Well, to keep the bucket of water swinging in a curved arc, the centripetal force can be felt in the tension experienced by the handle (or your arm, in our case). Swinging an object around on a string will cause the rope to undergo tension (centripetal force), and if your rope isn’t strong enough, it will snap and break, sending the mass flying off in a tangent (straight) line until gravity and drag force pull the object to a stop. This force is proportional to the square of the speed – the faster you swing the object, the higher the force.


Centrifugal (translation = “center-fleeing”) force has two different definitions, which also causes confusion. The inertial centrifugal force is the most widely referred to, and is purely mathematical, having to do with calculating kinetic forces using reference frames, and is used with Newton’s laws of motion. It’s often referred to as the ‘fictitious force’.


Reactive centrifugal force happens when objects move in a curved path. This force is actually the same magnitude as centripetal force, but in the opposite direction, and you can think of it as the reaction force to the centripetal force. Think of how you stand on the Earth… your weight pushes down on the Earth, and a reaction force (called the “normal” force) pushes up in reaction to your weight, keeping you from falling to the center of the Earth. A centrifugal governor (spinning masses that regulate the speed of an engine) and a centrifugal clutch (spinning disk with two masses separated by a spring inside) are examples of this kind of force in action.


Say WHAT?!?

Don’t worry if these ideas make your brain turn into a pretzel. Most college students take three courses in this before it makes sense to them. Here’s one more example: imagine driving a car along a banked turn. The road exerts a centripetal force on the car, keeping the car moving in a curved path (the “banked” turn). If you neglected to buckle your seat belt and the seats have a fresh coat of Armor-All (making them slippery), then as the car turns along the banked curve, you get “shoved” toward the door. But who pushed you? No one – your body wanted to continue in a straight line but the car keeps moving in your path, turning your body in a curve. The push of your weight on the door is the reactive centrifugal force, and the car pushing on you is the centripetal force.


What about the fictitious (inertial) centrifugal force? Well, if you imagine being inside the car as it is banking with the windows blacked out, you suddenly feel a magical ‘push’ toward the door away from the center of the bend. This “push” is the fictitious force invoked because the car’s motion and acceleration is hidden from you (the observer) in the reference frame moving within the car.


Okay, enough talk for now. Let’s make two acceleration-measuring instruments (called ‘accelerometers’) so you can jump, run, swing, and zoom around to find out how many g’s you can pull. You’re going to make a cork-accelerometer and a g-force ring, both of which are used by my students when I teach mechanical engineering dynamics.


Cork Accelerometer

Fill an empty soda bottle to the top with water. Modify the soda bottle cap as follows: attach a string 8-10″ long to a clean wine cork. Hot glue the free end of the string to the inside of the cap. Place the cork and string inside the bottle and screw on the top (try to eliminate the air bubbles). The cork should be free to bob around when you hold the bottle upside-down.



 
Download Student Worksheet & Exercises


To use the accelerometer: invert the bottle and try to make the cork move about. Remember – it is measuring acceleration, which is the change in speed. It will only move when your speed changes. You can do this experiment in a car while doing your other vehicle experiment: Why Bother With Seatbelts?. The trouble with this accelerometer is that there are no measurements you can take – it’s purely visual. This next activity is more accurate at measuring the number of g-s you pull in a sharp turn (whether in a vehicle or in a roller coaster!)


G-Force Ring Accelerometer

One more university-level gadget for demonstrating the fascinating world of physical dynamics. This quick homemade device roughly measures acceleration in “g’s”. We used it to measure the g-force on roller coasters at Six Flags Magic Mountain, and it worked just as well as the expensive ones you buy in scientific catalogs!



Get about a foot of tubing – the bigger the diameter, the easier it will be to read. Also get a barbed union (plastic barbs work just fine). Fill your tube halfway with COLORED water (it’s impossible to read when it’s clear). Blue, green, red… your choice of food dye additive. Make an O-shape using your barb to water-seal the junction. Grab hold of one side and hold the circle vertical, with the barb-end pointing to the sky. The water should fill the bottom half, and air fills the top half. Make sure there are equal amounts of water and air in your tube. Make a mark on the tube where the water meets the air with a black marker. This is your 0-g reading (relative, of course). No acceleration. Not a whole lot of fun.


Now, for your 1-g mark – measure up 45 degrees from the first mark. (If the top of the circle is 90 degrees, and the 0-g mark is zero degrees, find the halfway point and label it).


The 2-g mark is 22.5 degrees up from the 1-g mark.


3-g mark is 11.25 degrees up from the last mark. And 4-g is 5.6 degrees up from the last mark. (See a pattern? You can prove this mathematically in college, and it’s kind of fun to figure out!)


Now, next time mom drives around town, hold the tube in your hand so that the water line starts at the zero mark. When she pulls a turn, see how far it sloshes up and tell her how many g’s she pulled. We also used to have contests to see who could pull the most g’s while spinning in a circle. Have fun!


Advanced Students:

Advanced Students: Download your G-Force lab here.


Exercises 


  1. Which accelerometer was better at giving a visual representation of accelerating?
  2. Which one do you prefer? Why?
  3. What activity did you do that created the most acceleration?
  4. What does that tell you about acceleration?

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Balloon Racers

Thursday September 10, 2009

We’re going to experiment with Newton’s Third law by blowing up balloons and letting them rocket, race, and zoom all over the place. When you first blow up a balloon, you’re pressurizing the inside of the balloon by adding more air (from your lungs) into the balloon. Because the balloon is made of stretchy rubber (like a rubber band), the balloon wants to snap back into the smallest shape possible as soon as it gets the chance (which usually happens when the air escapes through the nozzle area). And you know what happens next – the air inside the balloon flows in one direction while the balloon zips off in the other.


Question: why does the balloon race all over the room? The answer is because of something called ‘thrust vectoring’, which means you can change the course of the balloon by angling the nozzle around. Think of the kick you’d feel if you tried to angle around a fire hose operating at full blast. That kick is what propels balloons and fighter aircraft into their aerobatic tricks.


We’re going to perform several experiments here, each time watching what’s happening so you get the feel for the Third Law. You will need to find:


  • balloons
  • string
  • wood skewer
  • two straws
  • four caps (like the tops of milk jugs, film canisters, or anything else round and plastic about the size of a quarter)
  • wooden clothespin
  • a piece of stiff cardboard (or four popsicle sticks)
  • hot glue gun

First, let’s experiment with the balloon. Here’s what you can do:


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1. Blow up the balloon (don’t tie it)


2. Let it go.


3. Wheeeee!


4. Tie one end of the string to a chair.


5. Blow up the balloon (don’t tie it).


6. Tape a straw to it so that one end of the straw is at the front of the balloon and the other is at the nozzle of the balloon.


7. Thread the string through the straw and pull the string tight across your room.


8. Let go. With a little bit of work (unless you got it the first time) you should be able to get the balloon to shoot about ten feet along the string.


This is a great demonstration of Newton’s Third Law – the air inside the balloon shoots one direction, and the balloon rockets in the opposite direction. It’s also a good opportunity to bring up some science history. Many folks used to believe that it would be impossible for something to go to the moon because once something got into space there would be no air for the rocket engine to push against and so the rocket could not “push” itself forward.


In other words, those folks would have said that a balloon shoots along the string because the air coming out of the balloon pushes against the air in the room. The balloon gets pushed forward. You now know that that’s silly! What makes the balloon move forward is the mere action of the air moving backward. Every action has an equal and opposite reaction.


Multi-Stage Balloon Rocket

You can create a multi-stage balloon rocket by adding a second balloon to the first just like you see here in the video:



Tie a length of string through the room, having at least twenty feet of clear length.  Thread two straws onto the string before securing the end.  Punch the bottoms out of two foam coffee cups and tape parallel to the threaded straws.


Blow up balloons while they are inside the cups, so they extend out either end.  When blowing up the second balloon, sandwich the untied end of the first inflated balloon between the second inflated balloon surface and inside the cup.  Hold the second balloon’s end with a clothespin and release!


Balloon Racecar

Now let’s use this information to create a balloon-powered racecar. You’ll need the rest of the items outlined above to build your racecar. NOTE: in the video, we’re using the popsicle sticks, but you can easily substitute in a sheet of stiff cardboard for the popsicle sticks. (Either one works great!)



Download Student Worksheet & Exercises


You now have a great grasp of Newton’s three laws and with it you understand a good deal about the way matter moves about on Earth and in space. Take a look around. Everything that moves or is moved follows Newton’s Laws.


In the next unit, we will get into Newton’s Third Law a little deeper when we discuss momentum and conservation of momentum by whacking things together *HARD*. But more on this later…


Exercises


  1. What is Newton’s Third Law of Motion?
  2. Why does the balloon stop along the string?

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Advanced students: Download your Balloon Racer Lab here.


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Rocket Ball Launcher

Thursday September 10, 2009

This is a satisfyingly simple activity with surprising results. Take a tennis ball and place it on top of a basketball… then release both at the same time.


Instant ball launcher!


You’ll find the top ball rockets off skyward while the lower ball hit the floor flat (without bouncing much, if at all). Now why is that? It’s easier to explain than you think…


Remember momentum? Momentum can be defined as inertia in motion. Something must be moving to have momentum. Momentum is how hard it is to get something to stop or to change directions. A moving train has a whole lot of momentum. A moving ping pong ball does not. You can easily stop a ping pong ball, even at high speeds. It is difficult, however, to stop a train even at low speeds.


Mathematically, momentum is mass times velocity, or Momentum=mv.


One of the basic laws of the universe is the conservation of momentum.  When objects smack into each other, the momentum that both objects have after the collision, is equal to the amount of momentum the objects had before the crash. Once the two balls hit the ground, all the larger ball’s momentum transferred to the smaller ball (plus the smaller ball had its own momentum, too!) and thus the smaller ball goes zooming to the sky.


Materials:


  • two balls, one significantly larger than the other

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Download Student Worksheet & Exercises


Do you see how using a massive object as the lower ball works to your advantage here?  What if you shrink the smaller ball even more, to say bouncy-ball size?  Momentum is mass times by velocity, and since you aren’t going to change the velocity much (unless you try this from the roof, which has its own issues), it’s the mass that you can really play around with to get the biggest change in your results. So for momentum to be conserved, after impact, the top ball had to have a much greater velocity to compensate for the lower ball ‘s velocity going to zero.


You can also try a small bouncy ball (about the size of a quarter) and a larger bouncy ball (tennis-ball size) and rest the small one on top of the large one. Hold upright as high as you can, then release. If the balls stay put (the small one stays on top of the larger) at impact, the energy transfer will create a SUPER high bounce for the small ball. (Note how high the larger ball bounces when dropped.)


What happens if you try THREE?


Read more about impulse here.


Exercises 


  1. What is the mathematical formula for momentum?
  2.  Explain momentum in words.
  3.  What happens to the momentum of the bottom ball in this experiment?

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Impulse & Momentum

Thursday September 10, 2009

This experiment is for advanced students.It’s time for the last lesson of mechanics. After all this time, you now have a good working knowledge of the rules that govern almost all movement on this planet and beyond!! This lesson we get to learn about things crashing into one another!! Isn’t physics fun?! We are going to learn about impulse and momentum.
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Here’s what you need:


  • two pennies
  • you
  • a wall
  • wagon and a skateboard
  • friends

Momentum

Momentum can be defined as inertia in motion. Something must be moving to have momentum. Momentum is how hard it is to get something to stop or to change directions. A moving train has a whole lot of momentum. A moving ping pong ball does not. You can easily stop a ping pong ball, even at high speeds. It is difficult, however, to stop a train even at low speeds. Mathematically, momentum is mass times velocity, or Momentum=mv. The heavier something is and/or the faster it’s moving the more momentum it has. The more momentum something has, the more force it takes to get it to change velocity and the more force it can apply if it hits something.


Impulse

Now let’s discuss impulse. Impulse is a measure of force and time. Remember, force is a push or a pull, right? Well, impulse is how much force is applied for how much time. Mathematically it’s impulse equals force x time or impulse = Ft.


Think about baseball. When you hit a baseball, do you just smack it with the bat or do you follow through with the swing? You follow through right? Do you see how impulse relates to your baseball swing? If you follow through with your swing, the bat stays in contact with the ball for a longer period of time. This causes the ball to go farther. Follow through is important in golf, bowling, tennis and many sports for the same reason. The longer the force is imparted, the farther and faster your ball will go.


More About Impulse and Momentum

Ok, let’s add impulse and momentum together and see what we get. Impulse changes momentum. If an object puts an impulse on another object, the momentum of both objects will change. If you continue to push on your stalled car, you will change the momentum of the car right? If you are riding your bike while not paying any attention and crash into the back of a parked car, you will put an impulse on the car and you and the car’s momentum will change. (As a kid, I did this pretty often. That’s what you get when you ride and wonder at the same time. Believe me when I tell you that my momentum changed a lot more than the car’s did!!)


In fact, there is a mathematical formula about this impulse and momentum thing. Impulse = change in momentum or Ft = change in mv. Force x time = mass x velocity. Does that sound familiar to anyone? It’s awfully similar to Newton’s second law (F=ma) isn’t it? In fact it’s the same thing.


**Alert, alert serious math here, feel free to skip this.**

F t = m v


Now if we divide both sides by “t” we get F=mv/t. Another way to say v is d/t (distance over time).


So now we have F=m(d/t)/t.


Those two “t’s” together are the same as t2 and d/t2 is a (acceleration). So what we have now is F=ma!


This Ft = mv is very important, in fact, it can save your life. Seat belts, air bags, crumple zones and other car safety features are based on this formula. When you want to shrink the force of impact, you want to increase the time the impact takes. This is called the collision time. The longer the collision time the longer it takes your momentum to come to zero. Here’s the math.


If you are in a 1000 kg vehicle moving at 30 km/h your momentum is 1000 x 30 or 30,000. Now, lets say you hit a brick wall so your momentum goes from 30,000 to 0 in .5 seconds.


Ft=mv so F(.5) = 30,000 so F= 60,000N! (N is for Newton which is a unit of force. It takes about 1 Newton to lift an apple so this car hits with the force of 60,000 apples! Talk about apple sauce!)


That’s gonna leave a mark! Now lets say that instead of hitting a brick wall you hit a mound of hay and so the impact takes 3 seconds.


Now the formula looks like this: F(3)=30,000 or F= 10,000N.


See the difference, 60,000N versus 10,000N of force. All those safety features, seat belts, helmets, air bags, are designed to increase how long it takes your momentum to come to zero. Newton’s laws to the rescue! Let’s do a couple of experiments here to help this information have more impact (pun intended!).


Quick Momentum Experiment

1. Find a wall.


2. Hit it with your bare fist. Take it easy, just hit it with enough force that you feel the impact.


3. Now put a pillow in front of the wall and hit it with about the same force as you hit it before.


4. With the pillow in front of the wall, you can hit it a little harder if you like but again, don’t go nuts!


What did the pillow do? It slowed the time of impact. Remember our formula Ft=mv. When the momentum of your moving fist struck the wall directly, the momentum was cut to zero instantly and so you felt enough force to hurt a bit. When the pillow was in the way it took longer for your momentum to come to zero. So you could hit the pillow fairly hard without feeling much force. Basically a bike helmet is like a pillow for you head. It slows the time of impact, so when you fall off your bike, there is much less force on your head. Just be glad your mom doesn’t make you wear a pillow on your head!


So let’s go back to momentum for a minute. Momentum is inertia in motion. It is how much force it takes to get something to slow down or change direction. One more concept I’d like to give you this month, is conservation of momentum. This is basically momentum equals momentum or mathematically mv=mv. (Momentum is mass times velocity.) When objects collide, the momentum that both objects have after the collision, is equal to the amount of momentum the objects had before the collision. Let’s take a look at this with this experiment.


Another Quick Momentum Experiment

1. Put one penny on the table.


2. Put another penny on the table about 6 inches away from the first one.


3. Now, slide one penny fairly fast towards the first penny.


4. What you want to have happen, is that the moving penny strikes (or gives impulse to) the stationary penny head on. The moving penny should stop and the stationary penny will move.


5. Now, try that with other coins. Make big ones hit small ones and vice versa. It’s also fun to put a line of 5 coins all touching one another. Then strike the end of the line with a moving penny.


This is conservation of momentum. If you were able to strike the penny head on, you should have seen that the penny that was moving, stopped, and the penny that was stationary moved with about the same speed of the original moving penny. Conservation of momentum is mv = mv. Once the moving penny struck the other, all the moving penny’s momentum transferred to the second penny. Since the pennies weighed the same, the v (velocity) of the first penny is transferred to the second penny and the second penny moves with the same velocity as the first penny. What happens if you use a quarter and a penny? Make the quarter strike the penny. That penny should really zip! Again mv=mv. The mass of the quarter is much greater then the mass of the penny. So for momentum to be conserved, after impact, the penny had to have a much greater velocity to compensate for its lower mass.


Mathematically it would look like this (the masses are not accurate to make the math easier to see.)


After collision Mass of Quarter x Velocity of Quarter = Mass of Penny x Velocity of Penny


5g x 10m/s = 1g x v


50 = 1 x v


50/1 = v


50m/s = v


or 5g x 10m/s = 1g x 50 m/s


50 momentum = 50 momentum


After the collision, the penny is moving at 50 m/s, 5 times faster then the quarter was moving because the penny is 5 times lighter then the quarter.


And ANOTHER Quick Momentum Experiment

wtdcorkDo this experiment again, but this time make the cork heavier. I wrapped mine in duct tape and then jammed a roll of electrical tape on the end. If you wanted to tape a golf ball to the cork or tape a bunch of change to it it would work as well. Just try to make the cork a good bit heavier than it was in the first place.


This is a great example of impulse as well as conservation of momentum. The impulse (Ft) is the baking soda and vinegar gas mixture creating enough pressure to force the cork off the bottle. According to Newton’s Third Law, the force from that impulse has an equal and opposite reaction, so the bus goes one way and the cork goes the other. Now take that information and combine it with what you now know about conservation of momentum. The impulse is equal for the cork and the bus, so which one is going to have more velocity due to it’s lighter mass? The cork. mv=mv. Just like the penny and the quarter, the lighter cork will go farther than the heavier bus. Now if you make the cork heavier and try it again, what will you see? Now the bus moves farther but the cork moves less far. Again, conservation of momentum, mv = mv.


Yet ANOTHER Easy Momentum Activity

1. Put the wagon and the skateboard close to one another.


2. Have one person sit on the skateboard while the other sits on the wagon.


3. Make sure the wheels are straight on the wagon and that the sidewalk is relatively free of stuff in the way.


4. Have one person give a good shove to the other person. Usually, it is easier for the person on the skateboard to push on the wagon. If this is true with your setup, then do it that way. Otherwise, do whatever is easiest.


5. Feel free to add more people or weight to the wagon and try it again.


Can you see how this and the one before it are really showing the same concept? Who went farther and faster? The lighter person on the lighter vehicle right? The impulse of the push was the same for both vehicles, so both vehicles had the same momentum. Momentum is mass and velocity so if the mass for both vehicles was the same, the speed would be the same. If the mass of one was more then the mass of the other, then the heavier one would move more slowly then the lighter one.


Highlights for Momentum & Impulse

Impulse is the amount of time a force is put on an object. How hard and how long something gets pushed or pulled.
Ft = Impulse. Impulse affects the momentum of an object.


Momentum is inertia in motion, how hard it is to get something to change directions or speed. Momentum = mv.


Conservation of momentum; mv = mv. If something hits something else the momentum of the objects before the collision will equal the momentum of the objects after the collision.


Here is another experiment in momentum: Newton’s Second Law.


Advanced students: Download your Momentum lab here.


Download Student Worksheet & Exercises


Exercises 


  1. What is momentum?
  2. What is impulse?
  3. What is the conservation of momentum?

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Circular Motion

Wednesday September 9, 2009
This experiment is for advanced students. Circular motion is a little different from straight-line motion in a few different ways. Objects that move in circles are roller coasters in a loop, satellites in orbit, DVDs spinning in a player, kids on a merry go round, solar systems rotating in the galaxy, making a left turn in your car, water through a coiled hose, and so much more.

Velocity is always tangent to the circle in the direction of the motion, and acceleration is always directed radially inward. Because of these two things, the acceleration that arises from traveling in a circle is called centripetal acceleration (a word created by Sir Isaac Newton). There’s no direct relationship between the acceleration and velocity vectors for a moving particle.

[am4show have='p9;p38;p90;p44;' guest_error='Guest error message' user_error='User error message' ] If you have a bucket of water and you’re swinging it around your head, in order to keep a bucket of water swinging in a circle, the centripetal force can be felt in the tension experienced by the handle. Swinging an object around on a string will cause the rope to undergo tension (centripetal force), and if your rope isn’t strong enough, it will snap and break, sending the mass flying off in a tangential straight line until gravity and drag force pull the object to a stop.

Here’s a cool experiment you can do that will really show you how objects that move in a circle experience centripetal force. You can lift at least 10 balls by using only one! All you need are balls, fishing line or dental floss, and an old pen.



Download Student Worksheet!

Remember Newton’s First Law? The law of inertia? It states that objects in motion tend to stay in motion with the same speed and direction unless acted upon by an unbalanced/external force. Which means that objects naturally want to continue going their straight and merry way (like you did in a straight line when you were inside the car) until an unbalanced force causes it to turn speed up or stop. Can you see how an unbalanced force is required for objects to move in a circle? There has to be a force pushing on the object, keeping in on a circular path because otherwise, it’ll go off in a straight line!

There's a whole section just on circular motion in our advanced section here. [/am4show]

Detecting the Gravitational Field

Friday August 14, 2009

Ok, sort of a silly experiment I admit. But here’s what we’re going for – there is an invisible force acting on you and the ball. As you will see in later lessons, things don’t change the way they are moving unless a force acts on them. When you jump, the force that we call gravity pulled you back to Earth. When you throw a ball, something invisible acted on the ball forcing it to slow down, turn around, and come back down. Without that force field, you and your ball would be heading out to space right now!
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Here’s what you need:


  • you
  • the Earth (or any planet that’s convenient)
  • a ball


Download Student Worksheet & Exercises


Here’s what you do:


1. Jump!
2. Carefully observe whether or not you come back down.
3. Take the ball and throw it up.
4. Again, watch carefully. Does it come down?


Gravity is probably the force field you are most familiar with. If you’ve ever dropped something on your foot you are painfully aware of this field! Even though we have known about this field for a looooong time, it still remains the most mysterious field of the four.


What we do know is that all bodies, from small atoms and molecules to gigantic stars, have a gravitational field. The more massive the body, the larger its gravitational field. As we said earlier, gravity is a very weak force, so a body really has to be quite massive (like moon or planet size) before it has much of a gravitational field. We also know that gravity fields are not choosy. They will attract anything to them.


All types of bodies, from poodles to Pluto, will will attract and be attracted to any other type of body. One of the strangest things about gravity is that it is only an attractive force. Gravity, as far as we can tell, only pulls things towards it. It does not push things away. All the other forces are both attractive (pull things towards them) and repulsive (push things away). (Gravity will be covered more deeply in a later lesson.)


Exercises 


  1. What did you determine about gravity and how it affects the rate of falling?
  2. Did changing the object affect the rate of falling? Why or why not?
  3. Did changing the variable affect the rate of falling?  Why or why not?

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Advanced students: Download your Gravitational Force Lab here.


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Detecting the Magnetic Field

Friday August 14, 2009

Remember, there are four different kinds of forces: strong nuclear force,
electromagnetism, weak nuclear force, and gravity. There are also four basic force fields that you come into contact with all the time. They are the gravitational field, the electric field, the magnetic field, and the electromagnetic field. Notice that those four force fields really only use two of the four different kinds of force: electromagnetism and gravity. Let’s take a quick look at what causes these four fields and what kind of objects they can affect, starting with the magnetic field.


Here’s what you need:
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  • you
  • a compass

You’re probably fairly familiar with magnetic fields. If you’ve ever stuck a magnet to a refrigerator, you’ve taken advantage of magnetic fields. Sticking a magnet to a refrigerator is one of those every day experiences that should just be absolutely flabbergasting. There you are holding an “I’d Rather be Relative” magnet and it sticks to the fridge! But wait a minute, if you put it on the wall… it falls off! How does it “know” what to stick to? Not only does it stick to the fridge, it also pushes some things away, attracts other things and couldn’t care less about still other things. What’s that all about?! We rarely think about what magnets do but, wow, the things they do are weird!


Magnetic fields come from objects that have a surplus of electrons all moving in the same direction. This can be an electric wire with current running through it or one of several special types of metals. Iron, nickel and cobalt are the most common metals that can be magnetic. Magnetic fields can only affect objects that can be magnetic themselves. That’s why a magnet can attract an iron nail, but it can’t attract an aluminum can. The iron nail can be magnetic, but the aluminum cannot. Magnets can also be attractive or repulsive. Two magnets with the same kind of poles facing one another will push themselves apart. Two magnets with opposite poles facing one another will pull themselves together.



 
Download Student Worksheet & Exercises


Using a compass and the Earth, you can do a simple experiment to detect the magnetic field of our planet. (If you don’t have a compass, just slide a magnet along the length of a needle several times (make sure you only swipe in one direction!) then stick it through a cork or bit of foam. Float the needle-foam thing in a cup of water.)


1. Look at the compass


2. Walk anywhere and keep your eye on the compass.


3. Turn around in circles and keep your eye on the compass (don’t get too dizzy).


Again a very simple little activity, but I hope you can see the point. No matter where you went or what you did, that needle always pointed the same direction! The Earth’s magnetic force field, another strange and mysterious force, always pushes that needle in the same direction. It’s invisible and you can’t feel it…but the needle can!


 Exercises 


  1. Why does the needle need the foam?
  2.  Why do we use water?
  3. What are the forces in a magnetic field?

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The Electromagnetic Field

Friday August 14, 2009

iStock_000002030797XSmallThe electromagnetic field is a bit strange. It is caused by either a magnetic field or an electric field moving. If a magnetic field moves, it creates an electric field. If an electric field moves, it creates a magnetic field.


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A moving electric field creates a moving magnetic field, which creates a moving electric field, which creates a moving magnetic field and on and on. Pretty strange huh? So an electromagnetic field is both an electric field and a magnetic field all rolled into one. Light, radio waves, and microwaves are examples of electromagnetic waves created by moving self-creating electronic and magnetic fields.


Until 1820, magnetic fields and electric fields were thought of as two completely separate things. A fellow by the name of Hans Christian Ørsted was preparing for a lecture when he noticed that a compass needle jumped when he flipped a switch that caused electricity to flow through some wires. This chance observation caused him to investigate further and discover that electric fields create magnetic fields and vice-versa.


Thus, through a completely random observation, electromagnetism was discovered. Without that discovery we wouldn’t have electric engines, radios, cell phones, television or Filbert the Flounder electric toothbrushes! Hooray for Ørsted!


A Summary of Force Fields and Objects

Type of Force Field Objects That Create Force Objects Are Affected By Force Force Field Can Attract and Repel
Gravitational Any object Any object Only attract
Magnetic Moving Electrons A metal that can be magnetic Attract and repel
Electric An electrically charged body Any body Attract and repel
Electromagnetic A moving magnetic field or a moving electric field A magnetic or electric field Attract and repel

The rest of this experiment (below) is for advanced students: (Hint: you need to have access to upper level content.)
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Measuring the Earth’s Magnetic Pulse

When you stare at a compass, the needle that indicates the magnetic field from the Earth appears to stand still, but we’re going to find how it fluctuates and moves by creating a super-sensitive instrument using everyday materials (for comparison, you would spend over $100 for a scientific instrument that does the same thing).


Today you get to learn how to amplify tiny pulses in the Earth’s magnetic field using a laser and a couple of magnets. It’s a very cool experiment, but it does take patience to make it work right.


Materials


  • Index card or scrap of cardboard
  • 2 small mirrors
  • 2 rare earth magnets
  • Nylon filament (thin nylon thread works, too)
  • 4 doughnut magnets
  • Laser pointer (any kind will work – even the cheap key-chain type)
  • Clean glass jar (pickle, jam, mayo, etc… any kind of jar that’s heavy so it won’t knock over easily)
  • Wooden spring-type clothespin
  • Hot glue gun, scissors and tape


Download Student Worksheet & Exercises


  1. Sandwich the twine between the two rare earth magnets. These are the stronger magnets.
  2. Use a tiny dab of glue on one of the magnets and attach a mirror to the magnet. Do this on the other side for the second magnet and mirror.
  3. Lower the mirror-magnets into the container, leaving it hanging an inch above the bottom of the jar. Cut the twine at the mouth level of the container.
  4. Glue the top of the twine to the bottom of the lid, right in the center.
  5. When the glue has dried, place your mirror-magnets inside the jar and close the lid. Make sure that the mirror-magnets don’t touch the side of the jar, and are free to rotate and move.
  6. You’ve just built a compass! The small magnets will align with the Earth’s magnetic field. Slowly rotate the jar, and watch to see that the mirror-magnets inside always stay in the same configuration, just like the needle of a standard compass.
  7. Set your new compass aside and don’t touch it. You want the mirror-magnets to settle down and get very still.
  8. You are going to build the magnet array now. Stack your four doughnut magnets together in a tall stack.
  9. Fold your index card in half, and then open it back up. On one side of the crease you’re going to glue your magnets. When the magnets are attached, you’ll fold the card over so that it sits on the table like a greeting card with the magnets facing your glass jar.
  10. Tape your index card down to the table as you build your magnet array. (Otherwise the paper will jump up mid-way through and ruin your gluing while you are working.)
  11. Place a strip of glue on the bottom magnet of your stack and press it down onto the paper, gluing it into place.
  12. Lift the stack off (the bottom magnet should stay put on the paper) and place glue on the bottom magnet. Glue this one next to the first.
  13. Continue with the array so you have a rectangle (or square) arrangements of magnets with their poles oriented the same way. Don’t flip the magnets as you glue them, or you’ll have to start over to make sure they are lined up right.
    Since we live in a gigantic magnetic field that is 10,000 times more powerful than what the instrument is designed to measure, we have to “zero out” the instrument. It’s like using the “tare” or “zero” function on a scale. When you put a box on a scale and push “zero”, then the scale reads zero so it only measures what you put in the box, not including the weight of the box, because it’s subtracting the weight of the box out of the measurement. That’s what we’re going to do with our instrument: we need to subtract out the Earth’s magnetic field so we just get the tiny fluctuations in the field.
  14. Place your instrument away from anything that might affect it, like magnets or anything made from metal.
  15. Fold the card back in half and stand it on the table. We’re normally going to keep the array away from the jar, or the magnet array will influence the mirror-magnets just like bringing a magnet close to a compass does. But to zero out our instrument, we need to figure out how far away the array needs to be in order to cancel out the Earth’s field.
  16. Bring the array close to your jar. You should see the mirror-magnets align with the array.
  17. Slowly pull the magnet array away from the compass to a point where if it were any closer, the mirror-magnets would start to follow it, but any further away and nothing happens. It’s about 12 inches away. Measure this for your experiment and write it on your array or jar so you can quickly realign if needed in the future.
  18. Insert your laser pointer into the clothespin so that the jaws push the button and keep the laser on. Place it at least the same distance away as the array. You might have to prop the laser up on something to get the height just right so you can aim the laser so that it hits the mirror inside. (Note that you’ll have a reflection from the glass as well, but it won’t be nearly as bright.)
  19. Find where the laser beam is reflected off the mirror and hits the wall in your room. Walk over and tape a sheet of paper so that the dot is in the middle of the paper. Use a pen and draw right on top of the dot, and mark it with today’s date.
  20. Do you notice if it moves or it is stays put? Sometimes the dot will move over time, and other times the dot will wiggle and move back and forth. The wiggles will last a couple of seconds to a couple of minutes, and those are the oscillations and fluctuations you are looking for!
  21. Tape a ruler next to the dot so you can measure the amount of motion that the dot makes. Does it move a lot or a little when it wiggles? Two inches or six?

What’s Going On?

The reason this project works is because of tiny magnetic disturbances caused by the ripples in the ionosphere. Although these disturbances happen all the time and on a very small scale (usually only 1/10,000th of the Earth’s magnetism strength), we’ll be able to pick them up using this incredibly simple project. Your reflected laser beam acts like an amplifier and picks up the movement from the magnet in the glass.


Construction tip for experiment:
You need to use a filament that doesn’t care how hot or humid it is outside, so using one of the hairs from your head definitely won’t work. Cotton tends to be too stretchy as well. Professionals use fine quartz fibers (which are amazingly strong and really don’t care about temperature or humidity). Try extracting a single filament from a multi-stranded nylon twine length about 30″ long. If you happen to have a fine selection of nylon twine handy, grab the one that is about 25 microns (0.01″) thick. Otherwise, just get the thinnest one you can find.


You can tape a wooden clothespin down to the table and insert your laser pointer inside – the jaws will push the button of the laser down so you can watch your instrument and take your measurements. When you’re ready, tape a sheet of paper to the wall where your reflected beam (reflected from the mirror, not the glass… there will be two reflected beams!) hits the wall and mark where it hits. Over periods of seconds to minutes, you’ll see deflections and oscillations (wiggles back and forth) – you are taking the Earth’s magnetic pulse!


In order for this experiment to work properly, ALL magnets (including the penny described below) need to be in the same plane. That is, they all need to be the same height from the ground. You can, of course, rotate the entire setup 90 degrees to investigate the magnetic ripples in the other planes as well!


To make this instrument even more sensitive, glue a copper penny (make sure it’s minted before 1982, or you’ll get an alloy, not copper, penny) to the glass jar just behind the magnets (opposite the laser). When your magnets move now, they will induce eddy currents in the penny that will induce a (small) magnetic field opposite of the rotation of the magnets to dampen out “noise” oscillation. In short, add a penny to the glass to make your instrument easier to read.


Also, note that big, powerful magnets will not respond quickly, so you need a lightweight, powerful magnet.


You can walk around with your new instrument and you’ll find that it’s as accurate as a compass and will indicate north. You probably won’t see much oscillation as you do this. Because the Earth has a large magnetic field, you have to “tare” the instrument (set it to “zero”) so it can show you the smaller stuff. Use the doughnut magnets about 30 centimeters away as shown in the video.


Exercises


  1. Does the instrument work without the magnet array?
  2. Why did we use the stronger magnets inside the instrument?
  3. Which planet would this instrument probably not work on?

Advanced Students: Download your Electromagnetic Field Lab here.


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Detecting the Electric Field

Friday August 14, 2009

You are actually fairly familiar with electric fields too, but you may not know it. Have you ever rubbed your feet against the floor and then shocked your brother or sister? Have you ever zipped down a plastic slide and noticed that your hair is sticking straight up when you get to the bottom? Both phenomena are caused by electric fields and they are everywhere!


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An electric field exists when at least one body is electrically charged. Atoms are filled with positively charged protons and negatively charged electrons. If an object has more electrons than protons, it will be negatively charged and if it has fewer electrons than protons, it will be positively charged. Electric fields, like magnetic fields, can attract and repel. If two bodies have the same kind of charge, that is either both are negative or both are positive, they will push themselves away from each other. If one body has a positive charge and the other has a negative charge, they will attract each other. Charged bodies can also attract bodies that are neither positive nor negative but are just neutral.


Electric fields are extremely common. If you comb your hair with a plastic comb, you cause that comb to have a small electric field. When you take off a fleece jacket or a polyester sweat shirt, you create an electric field that may be thousands of volts! Don’t worry, you can’t get hurt. There may be lots of voltage but there will be very little amperage. It’s the amperage that actually hurts you.


Here’s a simple experiment you can do that only needs four simple items:
– head of hair
– balloon
– yardstick or meterstick
– large spoon


Here’s what you do:



 
Download Student Worksheet & Exercises


Make sure you’ve tried out these Static Electricity experiments and learn how to light a bulb without plugging it into the wall!


Exercises 


  1. What happens if you rub the balloon on other things, like a wool sweater?
  2. If you position other people with charged balloons around the table, can you keep the yardstick going?
  3.  Can we see electrons?
  4.  How do you get rid of extra electrons?
  5.  Does the shape of the balloon matter?
  6.  Does hair color matter?
  7.  Rub a balloon on your head, and then lift it up about 6”. Why is the hair attracted to the balloon?
  8. Why does the hair continue to stand on end after the balloon is taken away?
  9. What other things does the balloon stick to besides the wall?
  10. Why do you think the yardstick moved?
  11. What other things are attracted or repelled the same way by the balloon? (Hint: try a ping pong ball.)

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Flying Paper Clip

Friday August 14, 2009

Have you ever been close to something that smells bad? Have you noticed that the farther you get from that something, the less it smells, and the closer you get, the more it smells? Well forces sort of work in the same way.


Forces behave according to a fancy law called the inverse-square law. To be technical, an inverse-square law is any physical law stating that some physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity.


The inverse-square law applies to quite a few phenomena in physics. When it comes to forces, it basically means that the closer an object comes to the source of a force, the stronger that force will be on that object. The farther that same object gets from the force’s source, the weaker the effect of the force.


Mathematically we can say that doubling the distance between the object and the source of the force makes the force 1/4th as strong. Tripling the distance makes the force 1/9th as strong. Let’s play with this idea a bit.


Here’s what you need:


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  • magnet
  • paper clip
  • string
  • tape

Using a magnet (the stronger the better), paper clip, string (or yarn or even dental floss!), and tape, you can make a flying paper clip.


OPTIONAL: If you happen to have a spring scale and ruler, get those out, too…otherwise, just skip these items – they are not essential to understanding the concept here.



 
Download Student Worksheet & Exercises


1. Tie about 4 inches of string to a paper clip.
2. Tape the magnet to the table.
3. Hold the end of the string that is not tied to the paper clip and let the paper clip dangle.
4. Slowly bring the paper clip closer and closer to the magnet.
5. Notice that the closer you get to the magnet, the stronger the force of the magnetic field is on the paper clip.


If you have a spring scale:
6. Attach the paper clip to the spring scale.
7. Move the paper clip closer to the magnet until the magnetic field affects the paper clip.
8. Measure how far the paper clip is from the magnet with a ruler.
9. Measure how much pull there is on the paper clip. Use newtons if your spring scale shows that measurement, but grams are OK if it doesn’t.
10. Bring the paper clip a half inch closer and measure the force of the pull again either in grams or Newtons.
11. Continue to get closer to the magnet half an inch at a time, measuring the force until you can’t get any closer.


What you may have noticed here was that the closer you got the paper clip to the magnet (the object causing the force field) the stronger the force was on the paper clip. You have just seen the inverse-square law in action!


Exercises: 


  1. Circle one: The closer you get to the magnet, the (stronger | weaker) the force of the magnetic field is on the paper clip.
  2. Why does it matter which way you orient the magnet in this experiment?
  3. Which magnet has the strongest magnetic field?
  4. Is the north or south pole stronger on a magnet?

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Force-full Cereal

Friday August 14, 2009

cerealDid you know that your cereal may be magnetic? Depending on the brand of cereal you enjoy in the morning, you’ll be able to see the magnetic effects right in your bowl. You don’t have to eat this experiment when you’re done, but you may if you want to (this is one of the ONLY times I’m going to allow you do eat what you experiment with!) For a variation, pull out all the different boxes of cereal in your cupboard and see which has the greatest magnetic attraction.


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Materials:


  • a bowl of cereal with milk
  • spoon


 
Download Student Worksheet & Exercises


  1. Fill the bowl with milk.
  2. Put about 20 pieces of cereal (not the whole box!) into the bowl.
  3. Stir up the bowl a little and watch what happens.

If you watched carefully, you saw that as the cereal “O’s” got close to one another, they attracted each other. The closer they got, the stronger was their attraction to each other and the faster they moved towards each other. If you wait and watch long enough, you get a nice tight batch of cereal all clustered together in one or two big blobs. This activity is a great illustration of what is meant by the inverse square law because the attraction between “O’s” was stronger the closer they got to each other.


I discovered this activity one morning as I was eating cereal. The same thing happens with bubbles when you’re doing the dishes. Science is everywhere! Feel free to eat the cereal!


Exercises 


  1. Why do the pieces of cereal stick to each other?
  2.  Does the cereal move slower or faster the closer the pieces come in contact with each other?
  3.  What other cereals does it work for?

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Net Forces

Friday August 14, 2009

It is very rare, especially on Earth, to have an object that is experiencing force from only one direction. A bicycle rider has the force of air friction pushing against him. He has to fight against the friction between the gears and the wheels. He has gravity pulling down on him. His muscles are pushing and pulling inside him and so on and so on.


Even as you sit there, you have at least two forces pushing and pulling on you. The force of gravity is pulling you to the center of the Earth. The chair is pushing up on you so you don’t go to the center of the Earth. So with all these forces pushing and pulling, how do you keep track of them all? That’s where net force comes in.


The net force is when you add up all of the forces on something and see what direction the overall force pushes in. The word “net”, in this case, is like net worth or net income. It’s a mathematical concept of what is left after everything that applies is added and subtracted. The next activity will make this clearer.


Here’s what you need:


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  • a rope (at least 3 feet long is good)
  • a friend
  • a sense of caution

(Be careful with this. Don’t pull too hard and please don’t let go of the rope. This is fun but you can get hurt if you get silly.)



Download Student Worksheet Exercises


1. You and your friend each grab an opposite end of the rope.
2. Both of you pull just a bit on the rope.
3. Have your friend pull a bit harder than you. Notice the direction that you both move.
4. Now you pull harder than your friend. Now which way do you go?
5. Lastly, both of you pull with the same strength on the rope. Even though you are both pulling, neither one of you should move.


In this experiment, there were always at least three forces pulling on the rope. Can you think of the three? They are you, your friend and gravity. You were pulling in one direction. Your friend was pulling in another direction and gravity was pulling down.


When one of you pulled harder (put more force on the rope) than the other person, there was a net force in the direction of the stronger pull. The rope and you guys went in that direction.


When both of you were pulling the same amount, there was an equal force pulling the rope one way and another equal force pulling the rope the other way. Since there were two equal forces acting in two opposite directions, the net force equaled zero, so there was no movement in either direction. No net force in this case means no movement.


As you’ll see when you learn about Newton’s second law, no net force means no acceleration.


Let’s take another look at the bicycle rider we talked about earlier. To make things easier, we’ll call him Billy. For Billy to speed up, he needs to win the tug of war between all of the forces involved in riding a bicycle. In other words, his muscles need to put more force on the forward motion of the bike than all of the forces of friction that are pushing against him.


If he wants to slow down, he needs to allow the forces of friction to win the tug of war so that they will cancel out his forward motion and slow down the bike. If he wants to ride at a steady speed, he wants the tug of war to be tied. His muscles need to exert the same amount of force pushing forward as the friction forces pulling in other directions.


Advanced Students: Download your Net Forces Lab here.


Exercises:


  1. For scenario 1, in which direction did you both move?  Draw the free body diagram below
  2. For scenario 2, in which direction did you both move?  Draw the free body diagram below
  3.  For scenario 3, in which direction did you both move?  Draw the free body diagram below
  4. For scenario 4, in which direction did the rope move?  Draw the free body diagram below
  5. What was the same about question 1 and question 4?  What was different?
  6.  Even though the forces were less in question 1 than question 4, what was the net force for both?
  7.  There were always at least 3 forces acting on the rope, what were they?  Did you include the third force in your free body diagram?
  8. If the rope was slightly off the ground, with gravity pulling it down, what does that tell you about the force you and your friend exerted?

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Building Bridges

Friday August 14, 2009

What keeps building from toppling over in the wind? Why are some earthquake-proof and others not? We’re going to look at how engineers design buildings and bridges while making our own.


Here’s what you need:
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  • Index cards
  • Blocks
  • Straws
  • Clay
  • Disposable cups

Watch the video:




Download Student Worksheet Exercises


Exercises 


  1. What are three different kinds of forces?
  2.  Using only blocks, what kind of wall design is the weakest?
  3.  Why does the bridge seem stronger when a card is arched underneath?

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Barrel Roof

Friday August 14, 2009

This roof can support over 400 times its own weight, and you don’t need tape! One of the great things about net forces is that although the objects can be under tremendous force, nothing moves! For every push, there’s an equal and opposite pull (or set of pulls) that cancel each other out.


This barrel roof is an excellent example of how to the forces all cancel out and the roof stands strong (hopefully!) If you have trouble with this experiment, just use cardstock or other heavy weight paper instead of regular copy paper.


Here’s what you need:
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Watch the video:



 
Download Student Worksheet & Exercises


Exercises 


  1. What is Newton’s Third Law?
  2. What kind of groups do forces come in?
  3. What is another name for Newton’s Third Law?

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A Weighty Issue

Wednesday August 12, 2009

This lesson may give you a sinking sensation but don’t worry about it. It’s only because we’re talking about gravity. You can’t go anywhere without gravity. Even though we deal with gravity on a constant basis, there are several misconceptions about it. Let’s get to an experiment right away and I’ll show you what I mean.

If I drop a ping pong ball and a golf ball from the same height, which one hits the ground first? How about a bowling ball and a marble?

Here's what you need:

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  • ping pong ball
  • golf ball
  • you

Download Student Worksheet & Exercises

For this experiment, you'll need:

Two objects of different weights. A marble and a golf ball, or a tennis ball and a penny for example.
A sharp eye
A partner

1. Take a careful look at both objects and make a prediction about which object will hit the ground first if they are dropped from the same height.

2. Test your prediction. Hold both objects at the same height. Make sure the bottom of both objects are the same distance from the floor.

3. Let them go as close to the same time as possible. Sometimes it's helpful to roll them off a book.

4. Watch carefully. Which hits the ground first, the heavier one or the lighter one? Try it a couple of times and watch carefully. It will be a little easier for the person who isn’t dropping them to see what happens.

What you should see is that both objects hit the ground at the same time! Gravity accelerates both items equally and they hit the ground at the same time. Any two objects will do this, a brick and a Buick, a flower and a fish, a kumquat and a cow!

“But,” I hear you saying, “if I drop a feather and a flounder, the flounder will hit first every time!” Ok, you got me there. There is one thing that will change the results and that is air resistance.

The bigger, lighter and fluffier something is, the more air resistance can effect it and so it will fall more slowly. Air resistance is a type of friction which we will be talking about later. In fact, if you removed air resistance, a feather and a flounder would hit the ground at the same time!!!

Where can you remove air resistance? The moon!!! One of the Apollo missions actually did this (well, they didn’t use a flounder they used a hammer). An astronaut dropped a feather and a hammer at the same time and indeed, both fell at the same rate of speed and hit the surface of the moon at the same time.


Ask someone this question: Which will hit the ground first, if dropped from the same height, a bowling ball or a tennis ball? Most will say the bowling ball. In fact, if you asked yourself that question 5 minutes ago, would you have gotten it right? It’s conventional wisdom to think that the heavier object falls faster. Unfortunately, conventional wisdom isn’t always right. Gravity accelerates all things equally. In other words, gravity makes all things speed up or slow down at the same rate. We will be discussing acceleration more in a later lesson. If you would like more details on the math of this, it will be at the end of this lesson in the Deeper Lesson section.

This photo shows a statue of Aristotle, a famous Greek philosopher who contributed many ideas to science.

This is a great example of why the scientific method is such a cool thing. Many, many years ago, there was a man of great knowledge and wisdom named Aristotle. Whatever he said, most people believed to be true. The trouble was he didn’t test everything that he said. One of his statements was that objects with greater weight fall faster than objects with less weight. Everyone believed that this was true.

Hundreds of years later Galileo came along and said “Ya know...that doesn’t seem to work that way. I’m going to test it” The story goes that Galileo grabbed a melon and an orange and went to the top of the Leaning Tower of Pisa. He said, “Look out below!” and dropped them! By doing that, he showed that objects fall at the same rate of speed no matter what their size.

It is true that it was Galileo who “proved” that gravity accelerates all things equally no matter what their weight, but there is no real evidence that he actually used the Leaning Tower of Pisa to do it.

Advanced Students: Download your Gravity Lab here.

Exercises

  1. What did you notice from your data? Did heavier or lighter objects fall faster? Did more massive objects or smaller objects fall faster? What characteristic seemed to matter the most?
  2. Is gravity a two-way force, like the attractive-repulsive forces of a magnet?
  3. If I were to drop a bowling ball and a balloon filled with a gas six times heavier than air (sulfur hexafluoride SF6) and inflated to the exact size of the bowling ball from my roof, which will strike the ground first?

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Forever Falling

Wednesday August 12, 2009

If I toss a ball horizontally at the exact same instant that I drop another one from my other hand, which one reaches the ground first? For this experiment, you need: [am4show have=’p8;p9;p11;p38;p72;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • 2 rulers or paint sticks. Any thing wide and flat
  • 2 coins or poker chips
  • A sharp eye and ear
  • A partner is good for this one too


 


Download Student Worksheet & Exercises
1. Place one of the rulers flat so that it is diagonal across the edge of a table with half the ruler on the table and half sticking off.


2. Place one coin on the table, just in front of the ruler and just behind the edge of the table. Place the other coin on the ruler on the side where it’s off the table.


3. Put your finger right in the middle of the ruler on the table so that you are holding it in such a way that it can spin a bit under your finger. Now with the other ruler you are going to smack the end of the first ruler so that the first ruler pushes the coin off the desk and the coin that’s resting on the ruler falls to the ground.


4. Now, before you smack the ruler, make a prediction. Will the coin that falls straight down or the coin that is flying forward hit the ground first?


5. Try it. Do the test and look and listen carefully to what happens. It’s almost better to use your ears here than your eyes. Do it a couple of times.


Are you surprised by what you see and/or hear? Most people are. It’s not what you would expect.


The coins hit the ground at the SAME time. Is that odd or what?


bullet


Did you read the first sentence at the top of this lab? What do you think will happen?


The balls will hit the ground at the exact SAME time.


Gravity doesn’t care if something is moving horizontally or not. Everything falls toward the center of the Earth at the same rate.


Let me give you a better example: A bullet fired parallel to the ground from a gun and a bullet dropped from the same height at the same time will both hit the ground at the same time. Even though the one fired lands a mile away! It seems incredible, but it’s true.


Gravity doesn’t care what size something is or whether or not it is moving, Gravity treats all things equally and accelerates them the same.


Notice, that I say gravity accelerates all things equally, not gravity pulls on all things equally. Gravity does pull harder on some things than on other things. This is why I weigh more than a dog. I am made of more stuff (I have more atoms) than the average dog, so gravity pulls on me more.


Weight is nothing more than a measure of how much gravity is pulling on you. This is why you can be “weightless” on a scale in space. You are still made of stuff, but there’s a balance of the gravity that is pulling on you and the outward force due to the acceleration since you’re moving in a circle (which you do in order to remain in orbit), so it feels like you have no weight.


The larger a body is, the more gravitational pull or the larger a gravitational field it will have.


The Moon has a fairly small gravitational field (if you weighed 100 pounds on Earth, you’d only be 17 pounds on the Moon), the Earth’s field is fairly large and the Sun has a HUGE gravitational field (if you weighed 100 pounds on Earth, you’d weigh 2,500 pounds on the sun!).


As a matter of fact, both the dog and I both have gravitational fields! Since we are both bodies of mass we have a gravitational field which will pull things towards us. All bodies have a gravitational field. However, my mass is sooooo small that the gravitational field I have is miniscule. Something has to be very massive before it has a gravitational field that noticeably attracts another body.


So what’s the measurement for how much stuff you’re made of? Mass. Mass is basically a weightless measure of how much matter makes you, you. A hamster is made of a fairly small amount of stuff so she has a small mass. I am made of more stuff, so my mass is greater than the hamster’s. Your house is made of even more stuff so its mass is greater still.


So, here’s a question. If you are “weightless” in space, do you still have mass? Yes, the amount of stuff you’re made of is the same on Earth as it is in your space ship. Mass does not change but since weight is a measure for how much gravity is pulling on you, weight will change.


Did you notice that I put weightless in quotation marks? Wonder why?


Weightlessness is a myth! Believe it or not, one is never weightless. A person can be pretty close to weightless in very deep space but the astronauts in a space ship actually do have a bit of weight.


Think about it for a second. If a space ship is orbiting the Earth what is it doing? It’s constantly falling! If it wasn’t moving forward at 10’s of thousands of miles an hour it would hit the Earth. It’s moving fast enough to fall around the curvature of the Earth as it falls but, indeed, it’s falling as the Earth’s gravity is pulling it to us.


Otherwise the ship would float out to space. So what is the astronaut doing? She’s falling too! The astronaut and the space ship are both falling to the Earth at the same rate of speed and so the astronaut feels weightless in space. If you were in an elevator and the cable snapped, you and the elevator would fall to the Earth at the same rate of speed. You’d feel weightless! (Don’t try this at home!)


Exercises


  1. True or false? Gravity pulls on all things equally.
  2. True or false? Gravity accelerates all things equally.
  3. In your own words, why do the coins hit the ground at the same time? Is this what you’d expect to happen on Mars?

The rest of this experiment is for advanced students…[/am4show]


[am4show have=’p9;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]


For advanced students:

Either now, or at some point in the future you may ask yourself this question, “How can gravity pull harder (put more force on some things, like bowling balls) and yet accelerate all things equally?” When we get into Newton’s laws in a few lessons you’ll realize that doesn’t make any sense at all. More force equals more acceleration is basically Newton’s Second law.


Well, I don’t want to take too much time here since this is a little deeper then we need to go but I do feel some explanation is in order to avoid future confusion. The explanation for this is inertia. When we get to Newton’s First law we will discuss inertia. Inertia is basically how much force is needed to get something to move or stop moving.


Now, lets get back to gravity and acceleration. Let’s take a look at a bowling ball and a golf ball. Gravity puts more force on the bowling ball than on the golf ball. Soooo the bowling ball should accelerate faster since there’s more force on it. However, the bowling ball is heavier soooo it is harder to get it moving. Vice versa, the golf ball has less force pulling on it but it’s easier to get moving. Do you see it? The force and inertia thing equal out so that all things accelerate due to gravity at the same rate of speed!


Gravity had to be one of the first scientific discoveries. Whoever the first guy was to drop a rock on his foot, probably realized that things fall down! However, even though we have known about gravity for many many years, it still remains one of the most elusive mysteries of science. At this point, nobody knows what makes things move towards a body of mass.


Why did the rock drop towards the Earth and on that guy’s foot? We still don’t know. We know that it does, but we don’t know what causes a gravitational attraction between objects. Gravity is also a very weak force. Compared to magnetic forces and electrostatic forces, the gravitational force is extremely weak. How come? No one knows. A large amount of amazing brain power is being used to discover these mysteries of gravity. Maybe it will be you who figures this out!


Advanced students: Download your Forever Falling Lab here.


[/am4show]


Fast Ball

Wednesday August 12, 2009

You have just taken in a nice bunch of information about the wild world of gravity. This next section is for advanced students, who want to go even deeper. There’s a lot of great stuff here but there’s a lot of math as well. If you’re not a math person, feel free to pass this up. You’ll still have a nice understanding of the concept. However, I’d recommend giving it a try. There are some fun things to do and if you’re not careful, you might just end up enjoying it!


Here’s what you need:


[am4show have=’p9;p38;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • ball
  • pencil, paper
  • stopwatch
  • yardstick or tape measure


 
Download Student Worksheet & Exercises


Okay, let’s see where we can go here. Gravity accelerates all things equally…what does that mean? All things accelerate at 32 feet per second squared due to gravity. In metric, it accelerates 9.8 meters per second squared.


What that means is, every second something falls, its speed increases by 32 feet/second or 9.8 meters/second. Believe it or not, that’s about 22 miles per hour!! Gravity will accelerate something from 0 to 60 mph in about 3 seconds. Faster then all but the fastest sports cars!


So what is acceleration anyway? Well speed is the amount of distance something travels in a certain amount of time. Five miles per hour, for example, tells you that something can travel five miles in an hour. Acceleration is how much the speed changes over time. So acceleration would be miles per hour per hour or feet per second per second.


Acceleration is a rate of change of speed or, in other words, how fast is the speed is changing. Feet per second per second is the same as ft/s/s which is the same as ft/s². (I told you we were going deeper!) Let’s say you’re riding your bicycle at a positive acceleration (your getting faster) of 5 ft/s².


That means in 1 second you’re moving at a speed of 5 ft/s.


After 2 seconds you’re moving at a speed of 10 ft/s.


After 3 seconds you’re now clipping along at 15 ft/s (about 10 mph).


gravity1So you can see that as long as you accelerate, you will be getting faster and faster. The formula for this is v=at where v is velocity, a is acceleration and t is time. (We will be doing more with acceleration in a future lesson.)


If we want to find out how fast something is going after it has been dropped, we use the formula v=gt. The letter “v” stands for velocity (which basically means speed.) “g” stands for the gravitational constant and “t” stands for time.


If we want to find out how fast a golf ball is dropping after it falls for 3 seconds we multiply 3 seconds by 32 feet/second squared and that equals 96 feet/second. So, if I dropped a golf ball off a building, it would be going 96 feet per second after 3 seconds of dropping.


The formula looks like this when we fill in the numbers:


v=3s x 32 ft/s²


If we do more math, we’ll see that after one second something will be item7going 32 ft/s, after 2 seconds it will be going 64 ft/s, after 3 seconds 96 ft/s after 4 seconds 128 ft/s. Get it? Anything dropped will be going that speed after that many seconds because gravity accelerates all things equally (air resistance will effect these numbers so you won’t get exactly the numbers in practice that you will mathematically).


All right, lets go even deeper. We now know how to calculate how fast something will be going if it is dropped, but what happens if we throw it up? Well, which way does gravity go? Down right? Gravity accelerates all things equally so, gravity will slow things down as they travel up by 32 ft/s². If a ball is thrown up at 64 ft/s how long will it travel upwards? Well, since it is negatively accelerating (in physics there’s no such thing as deceleration) after the first second the ball will be traveling at 32 ft/s and after 2 seconds the ball will come to a stop, turn around in midair, and begin to accelerate downwards at 32 ft/s². Using this, you can tell how fast you can throw by using nothing more then a timer. Let’s try it.


For this experiment, you will need:


– A ball (a tennis ball or baseball would be perfect)


– A stopwatch


– Pencil and paper


– A friend


– A calculator


1. Go outside and pick one person to be the thrower and another to be the timer.
2. Have the timer say “Ready, Set, Go!” and at go he or she should start the stopwatch.
3. When the timer says go, the thrower should toss the ball as high as he or she can.
4. The timer should stop the stopwatch when the ball hits the ground.
5. Write down the time that the ball was in the air.
6. Let each person take a couple of turns as timer and thrower.
7. Now, come back inside and do a bit of math.


Ok, let’s see how you did. Let’s say you threw the ball into the air and it took 3 seconds to hit the ground. The first thing you have to do is divide 3 in half. Why? Because your ball traveled 1.5 seconds up and 1.5 seconds down! (By the way, this isn’t completely accurate because of two things. One, air resistance and two, the ball falls a little father then it rises because of the height of the thrower.) Now, take your formula and figure out the speed of the throw.


v=gt,


so v=32 ft/s² x 1.5 sec or


v = 48 ft/s.


So, if that’s how fast it left your hand…how fast was it going when it hit the ground? Yup, 48 ft/s. It has to be going the same speed because it had just as much time to speed up as it had to slow down, 1.5 seconds. Try that with your time and see how fast your throw was.


Ok, hold your breath, just a little deeper now. Let’s talk about distance. If something starts from rest you can tell how far it drops by how long it has dropped. This formula is d=1/2gt² or distance equals one half the gravitational constant multiplied by time squared. Let’s try it. If I drop a ball and it drops 3 seconds how far has it dropped?


d=1/2 32ft/s² x (3s)² or


d = 16 ft/s x 9s² or


d=144 ft So it has dropped 144 ft.


Now try this with your time. What’s the first thing you have to do? Divide your time in half again, right. It took your ball half the time to go up and half the time to come down. Now plug your numbers into 1/2gt² and find out how high you threw your ball! Is Major League Baseball in your future?!


Advanced students: Download your Fast Ball Lab here.


Exercises 


  1. Is gravity a speed, velocity, or acceleration?
  2. Does gravity pull equally on all things?
  3. Does gravity accelerate all objects equally?
  4. How is acceleration different from speed and velocity?

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Tracking your Treads

Monday August 10, 2009

Now let’s talk about the other ever present force on this Earth, and that’s friction. Friction is the force between one object rubbing against another object. Friction is what makes things slow down.


Without friction things would just keep moving unless they hit something else. Without friction, you would not be able to walk. Your feet would have nothing to push against and they would just slide backward all the time like you’re doing the moon walk.


Friction is a very complicated interaction between pressure and the type of materials that are touching one another. Let’s do a couple of experiments to get the hang of what friction is.
Here’s what you need:


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  • About 5 different shoes
  • A board, or a tray, or a large book at least 15 inches long and no more then 2 feet long.
  • A ruler
  • Paper
  • Pencil
  • A partner


 
Download Student Worksheet & Exercises


1. Put the board (or whatever you’re using) on the table.


2. Put the shoe on the board with the back of the shoe touching the back of the board.


3. Have a partner hold the ruler upright (so that the12 inches end is up and the 1 inch end is on the table) at the back of the board.


4. Slowly lift the back of the board leaving the front of the board on the table. (You’re making a ramp with the board). Eventually the shoe will begin to slide.


5. Stop moving the board when the shoe slides and measure the height that the back of the board was lifted to.


6. Look at the 5 shoes you chose and test them. Before you do, make a hypothesis for which shoe will have the most friction. Make a hypothesis. On a scale from 1 to 5 (or however many shoes you’re using) rate the shoes you picked. 1 is low friction and 5 would be high friction. Write the hypothesis next to a description of the shoes on a piece of paper. The greater the friction the higher the ramp has to be lifted. Test all of the shoes.


7. Analyze the shoes. Do the shoes with the most friction show any similarities? Are the bottoms made out of the same type of material? What about the shoes with very little friction?


Any surprises with which shoe had the most or least friction? Compare the shoe with the most friction and the shoe with the least friction. Do you notice anything? Usually, the shoe that has the most friction has more shoe surface touching the board then most of the other shoes.


Also, often the shoe with the least friction, has the least amount of shoe touching the board. Since friction is all about two things rubbing together, the more surface that’s rubbing, the more friction you get. A tire on you car should have treads but a race car tire will be absolutely flat with no treads at all. Why?


The race car doesn’t have to worry about rain or wetness so it wants every single bit of the tire to be touching the surface of the track. That way, there is as much friction as possible between the tire and the track. The tire on your car has treads to cut through mud and water to get to the nice firm road underneath. The treads actually give you less friction on a flat dry road!


Some of you might have used a skateboard shoe for your experiment. Notice, that the skateboard shoe has quite a flat bottom compared to most other shoes. This is because a skateboarder wants as much of his or her shoe to touch the board at all times.


Exercises 


  1. What is friction?
  2. What is static friction?
  3. What is kinetic friction?

[/am4show]


What a Drag!

Monday August 10, 2009
There’s a couple of misconceptions that I’d like to make sure get cleared up here a bit. I don’t want to go into too much detail but I want to make sure to mention these as they may be important as you go deeper into your physics education.

First, friction is not a fundamental force. Friction is actually caused by the elemental force of electro-magnetism between two objects.

Secondly, friction isn’t “caused” by the roughness or smoothness of an object. Friction is caused by two objects, believe it or not, chemically bonding to one another. Scientists call it “stick and slip”.

Think about it this way. When you pull the wood in this experiment, notice that the force needed to get the board moving was more then the force was to keep it moving. The surface you were pulling the board on never got any rougher or smoother, it stayed pretty much the same.

So why was it harder to get the board moving?

When the board is just sitting there, the chemical bonds between the board and the surface can be quite strong. When the board is moving however, the bonds are much weaker. Here's what you need:

[am4show have='p8;p9;p11;p38;p92;' guest_error='Guest error message' user_error='User error message' ]

  • A 6 inch long piece of 2 x 4 wood, or a heavy book
  • A string
  • A spring scale or a rubber band and a ruler.
  • Paper
  • Pen
  • 5 or so different surfaces, table tops, carpet, chairs, etc.




Download your worksheet here!

1. Write the different surfaces that you chose on a piece of paper.

2. Make a hypothesis. On a scale from 1 to 5 (or however many surfaces you chose) rate the surfaces you picked. 1 is low friction and 5 would be high friction. Write your ranking next to the surfaces on the paper.

3. Take your block or your book and attach a string to it.

4. Place your block on the surface to be tested.

5. If you have a spring scale, attach it to the string and carefully pull on your block until it just moves. What you will probably see, is that you will keep pulling and pulling until suddenly your block moves. Try to record the number that the scale said just before the block moved. It takes a little bit of practice to read that number so keep trying.

6. If you don’t have a spring scale, tie a rubber band to the string. Now put a ruler with the first inch at the end of the rubber band farthest from the block. Now pull on the rubber band holding it next to the ruler. When the block moves, record the number on the ruler where the end of the rubber band was. In other words, you are measuring how far the rubber band stretches before the board moves.

7. Remember, with the scale or the rubber band, this takes some getting used to so try not to get frustrated.

8. Write down your results next to your hypothesis.

9. The higher the number, the more friction there is between your board and the surface the board is on. In other words, the harder you had to pull to get the board moving, the more friction there is between the board and the surface.

10. Now analyze your data and see how the data matches your hypothesis. Which surface really had the most friction and which had the least. Write numbers 1 to 5 (or however many surfaces you chose) next to the results.

11. How did the data correlate with your hypothesis? Any surprises?

You’ve probably noticed with this experiment that the kind of surfaces rubbing together make a huge difference.

Flat, hard, smooth surfaces will have much less friction than a rubber, soft, or rough surface. Muddy, wet or icy surfaces will often have even less friction. So, if you remember what we talked about with shoes and tires, the job of the tread on a shoe and a tire is to cut through the lower friction water or mud and get down to the higher friction road or dry ground.

Something else I’d like you to notice is that friction acts differently depending on what something is doing. If you have ever had to push something heavy like a refrigerator you may have noticed that it was harder to get it to move than it was to keep it moving. This is because there are two types of friction; static friction and kinetic friction.

Static friction happens when something is resting on something else and not moving. Kinetic friction is when one thing is moving on something else. Static friction is usually greater than kinetic friction. This means that it is harder to get your fridge moving than to keep it moving. You may have noticed this during “What a Drag” (if not, go ahead and play with it some more). When you first got the board to move, your scale had measurements much higher than when it was actually moving. It was harder to get it moving than to keep it moving.

For the advanced students, here's a way to calculate the amount of force you're pulling with by figuring out how 'spring-y' your rubber band is...

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Advanced Students: Download your Friction Lab here. [/am4show]

Stick & Slip

Monday August 10, 2009

Friction is everywhere! Imagine what the world would be like without friction! Everything you do, from catching baseballs to eating hamburgers, to putting on shoes, friction is a part of it. If you take a quick look at friction, it is quite a simple concept of two things rubbing together.


However, when you take a closer look at it, it’s really quite complex. What kind of surfaces are rubbing together? How much of the surfaces are touching? And what’s the deal with this stick and slip thing anyway? Friction is a concept that’s many scientists are spending a lot of time on. Understanding friction is very important in making engines and machines run more efficiently and safely.


Here’s what you need:


[am4show have=’p8;p9;p11;p38;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • 2 Business card magnets (those thin flat magnets that are the size of business cards)
  • Fingers


 
Download Student Worksheet & Exercises


1. Take two business card magnets and stick them together black side to black side. They should be together so that the pictures (or whatever’s on the magnets) are on the outside like two pieces of bread on a sandwich.


2. Now grab the sides of the magnets and drag one to the right and the other to the left so that they still are magnetically stuck together as they slide over one another.


Did you notice what happened as they slid across one another? They stuck and slipped didn’t they? This is a bit like friction. As two surfaces slide across one another, they chemically bond and then break apart. Bond and break, bond and break as they slide. The magnets magnetically “bonded” together and then broke apart as you slide them across on another. (The chemical bonds don’t work quite like the magnetic “bonds” but it gives a decent model of what’s happening.) There are many mysteries and discoveries to be uncovered with this concept. Go out and make some!


Exercises 


  1. What is the difference between static and kinetic friction? Which one is always greater?
  2. Design an experiment where you can observe and/or measure the difference between static and kinetic friction.

[/am4show]


Exponential Friction

Monday August 10, 2009
expfrictionFind a smooth, cylindrical support column, such as those used to support open-air roofs for breezeways and outdoor hallways (check your local public school or local church). Wind a length of rope one time around the column, and pull on one end while three friends pull on the other in a tug-of-war fashion.

Experiment with the number of friends and the number of winds around the column. Can you hold your end with just two fingers against an entire team of football players? You bet!

[am4show have='p8;p9;p11;p38;p92;' guest_error='Guest error message' user_error='User error message' ] Here's what you need:

  • nylon long rope (10 feet or longer)
  • column or pillar (as talked about in the video)
  • at least two people, but more is better
What’s going on? This is a great example of what “exponential growth” truly means. There is friction between the rope and the support column – you can feel it as you tug on the rope. With every additional turn around the pole, the amount of friction increases (exponentially grows), until it skyrockets so much the rope feels as if it’s welded to the pole.

Download Student Worksheet & Exercises

Einstein himself stated that “exponential growth” was the eighth wonder of the world! Exercises 
  1. How much money would you earn on Day 20 if I gave you one penny on Day 1, and doubled it every day after so Day 2 you received 2 pennies, and Day 3 you got 4 pennies?
  2.  Why do you think this experiment with friction works? Does it work with a flat surface the same way as a curved surface?
[/am4show]

Bearings

Monday August 10, 2009

Stand on a cookie sheet or cutting board which is placed on the floor (find a smooth floor with no carpet). Ask someone to gently push you across the floor. Notice how much friction they feel as they try to push you.


Want to make this job a bit easier?


Here’s what you need:


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  • two boards (about 12″ x 12″, or whatever you have handy)
  • 4-10 dowels (or round, not hexagonal, pencils)
  • handful of marbles

Now place three or four dowels parallel about six inches apart between the board and the floor. Smooth wooden pencils can work in a pinch, as can the hard cardboard tubes from coat-hangers. Ask someone to push you. Is there a direction you still can’t travel easily? Now let’s add another direction to your motion…


Replace the dowels with marbles. What happens? Why do the marbles make you do in all directions? What direction(s) did the dowels roll you in?



 
Download Student Worksheet & Exercises


BONUS EXPERIMENT IDEA! To really drive this point home, you can make your own low-friction ball bearings: Get two cans (with a deep groove in the rim, such as paint cans) and stack them. Turn one (still on top of the other) and notice the resistance (friction) you feel. Now sandwich marbles all along the rim between the cans. Place a heavy book on top and note how easily it turns around. Oil the marbles (you can spray with cooking spray, but it is a bit messy) and it turns more easily yet.


Exercises 


  1. Why do the marbles make you go in all directions?
  2. What direction(s) did the dowels roll you in?

[/am4show]


Hovercraft

Monday August 10, 2009

hovercraftHovercraft transport people and their stuff across ice, grass, swamp, water, and land. Also known as the Air Cushioned Vehicle (ACV), these machines use air to greatly reduce the sliding friction between the bottom of the vehicle (the skirt) and the ground. This is a great example of how lubrication works – most people think of oil as the only way to reduce sliding friction, but gases work well if done right.


In this case, the readily-available air is shoved downward by the hover motor and the skirt traps the air and keeps it inside, thus lifting the vehicle slightly. The thruster motor’s job is to propel the craft forward. Most hovercraft use either two motors (one on each side) for steering, or just one with a rudder that can deflect the flow (as your project does).


The first hovercraft were thought about in the 1800s, but it wasn’t until the 1950s that real ones were first tested. Today, the military use them for patrolling hard-to-drive areas, scientists use them for swamp research studies, and businesses use them to transport toys and food across rough and icy areas. Scientists are already planning future ACVs to use magnetic levitation in addition to the air power… but it’s still on the drawing board.


Are you ready to make your own? We have TWO different models to choose from. Click this link for the Easy Balloon-Powered Model, or keep reading below for the advanced version.
[am4show have=’p8;p9;p11;p38;p72;p76;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]
You will need:


  • 1 wood skewer
  • 1 wood popsicle stick
  • 1 straw
  • 16 oz. styrofoam cup (the kind used for sodas). Note that waxed paper cups will not work!
  • 1 foam hamburger container (the one in the video is 5.5″ square and 3″ high when closed)
  • 1 foam meat tray (the one in the video is approx. 10″x12″x1″ – it does not need to be these exact dimensions – try a few different sizes out to see what happens! You can get them for free if you ask for a clean one from your butcher.
  • Two 3VDC motors (use this motor for the thruster and this motor for the hover motor)
  • 2  propellers (the ones in the video are 3″ diameter, so check your local hobby store and get a variety to test out) – read comments below for ideas on where to get props!
  • 9V battery clip with wires
  • 9V battery (get a good kind, like Duracell or Energizer)
  • 9V battery holder (looks like a “C”) OR use tape to attach the battery to your hovercraft
  • a couple of extra wires (speaker wire, alligator clips, etc.)
  • 1 SPST switch


Download Student Worksheet & Exercises


  1. First, we’ll work to make the hovercraft hover. Start by finding the center of the Styrofoam meat tray. This will be your base.
  2. Use the ruler to measure the diameter of your cup to make sure it’s 3.5 inches. If it measures correctly, use the cup and pen to draw a circle in the middle of the tray
  3. Carefully cut out the circle, supporting the bottom of the foam.
  4. Cut your skewer into three pieces, making sure they are longer than the cut-out circle is wide.
  5. Use the hot glue gun to attach the lip of the round motor onto the skewer pieces, keeping them as parallel as possible.
  6. Gently attach the skewers onto the foam.
  7. Attach a propeller onto the shaft of the motor which is now attached to the skewers and foam tray.
  8. Now we will work with the takeout container. Open it and cut it in half and place one half to the side.
  9. Check the diameter of the bottom of the foam cup to ensure it’s about 2 ¼ inches. Then you can trace it with a pen on the top of the hamburger container half.
  10. Cut out the circle and discard it.
  11. Using the slide switch as a guide, cut out a small rectangle in the front for the switch. Reinforce it with hot glue, being careful to NOT get hot glue in the switch. Make sure it still slides back and forth.
  12. Rest the hamburger half on top – we aren’t going to attach it just yet.
  13. Find the small motor and look for the small contacts (they are very small and fragile – they are copper and look a little like foil). Gently bend them up a little in the back.
  14. Hot glue the motor onto the end of the popsicle stick with the shaft pointing away from the stick and the contacts pointing up.
  15. Use hot glue to secure the stick across the top of the hole in the hamburger box.
  16. Attach a propeller and give it a spin to make sure it will spin.
  17. Find the 9-volt battery clip and hot glue the bottom of it onto the middle of the popsicle stick.
  18. Cut your wire into two equal length pieces. Remove the insulation from the ends (about ¾ of an inch to an inch – get adult help if you need it). Twist the exposed wires together. Do this for both wires.
  19. If you aren’t going to solder the project, you’ll need to cut off the metal ends of the 9 volt battery clip’s wires and strip the wire insulation. Twist these wires together as well.
  20. Now we’ll work on wiring the inside motor. Take the end of one wire and put it halfway through one of the posts. Bend it up and twist it around itself very well to ensure it’s connected well. Do this with the other wire and connection.
  21. One of these wires will go to the switch. Thread the wire through a tab and twist it around itself.
  22. Attach the black wire from your 9-volt battery clip to the other tab on the switch.
  23. Thread your remaining wires (the red one from the battery clip and the remaining red wire from the first, hovering motor) up through the hamburger tray to attach them to the second motor. This is the thruster motor.
  24. Now that everything is wired, glue the hamburger tray to the bottom tray by placing hot glue at each of the four corners and pressing down gently.
  25. To test, grab your 9-volt battery. Check to make sure everything is wired correctly – the hovercraft should hover, not be sucked down to the table, and you should feel air blowing if you hold your hand in front of the thruster motor. Switch the appropriate wires if you note any issues during testing.
  26. Now we’ll build a shroud around the thruster motor. You’ll need the cup, the last piece of wooden skewer, the straw, and the remaining big piece of foam. Measure about halfway down the cup and cut it all the way around – essentially cutting it in half. You’ll be using the top of the cup – the cuff-like portion. It should fit around the propeller.
  27. Starting on the cut side of the foam, cut out a rectangle to use as a shim. Hot glue the rectangle down to the hovercraft. Then hot glue the cup cuff down to the rectangle.
  28. If the propeller is hitting the Styrofoam, you can move the cup around and hot glue as needed to make sure there is room for movement.
  29. Make a vein from a rectangular piece of Styrofoam that fits inside the cup cuff.
  30. Glue the straw onto the long end of this piece and trim the straw down. The wooden skewer should fit right through the straw.
  31. Push the wooden skewer down through the top of the cup. Pierce the bottom of the cup but DO NOT pierce the bottom of the hovercraft.
  32. Put the straw and Styrofoam piece in, and then thread the skewer back down through the straw.
  33. Troubleshooting: make sure the bottom of the hovercraft – the tray’s lip – is as smooth as possible. You can sand it down lightly if you need to. You’ll need a clean, smooth, flat surface to hover on as well! You might also double check the motor directions. If necessary, you can lightly weigh down the front of the hovercraft to balance out the weight from the back.
  34. Modification: Once the hovercraft is operational, you can hot glue foam tubing to the bottom to make a water hovercraft. However, it will no longer work on land!

Exercises:


1.  What happens if you use a larger meat tray?


2.  Add another 9V battery?


3.  se a 12VDC motor for the 3VDC motor?


4.  Remove the battery pack entirely and add longer wires so you can hold the battery in your hand as the hovercraft zooms down the hallway?


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Buzzing Hornets

Monday July 27, 2009

hornet1Sound is everywhere. It can travel through solids, liquids, and gases, but it does so at different speeds. It can rustle through trees at 770 MPH (miles per hour), echo through the ocean at 3,270 MPH, and resonate through solid rock at 8,600 MPH.


Sound is made by things vibrating back and forth, whether it’s a guitar string, drum head, or clarinet. The back and forth motion of an object (like the drum head) creates a sound wave in the air that looks a lot like a ripple in a pond after you throw a rock in. It radiates outward, vibrating it’s neighboring air molecules until they are moving around, too. This chain reaction keeps happening until it reaches your ears, where your “sound detectors” pick up the vibration and works with your brain to turn it into sound.


You can illustrate this principle using a guitar string – when you pluck the string, your ears pick up a sound. If you have extra rubber bands, wrap them around an open shoebox to make a shoebox guitar. You can also cut a hole in the lid (image left) and use wooden pencils to lift the rubber band off the surface of the shoebox.


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Materials:


  • index card
  • rubber band
  • 3′ string
  • small piece of craft foam sheet OR a second index card
  • hot glue and glue sticks
  • tape


Download Student Worksheet & Exercises


Why is this happening? When you sling the hornet around, wind zips over the rubber band and causes it to vibrate like a guitar string… and the sound is focused (slightly) by the card. The card really helps keep the contraption at the correct angle to the wind so it continues to make the sound.


Troubleshooting: Most kids forget to put on the rubber band, as they get so excited about finishing this project that they grab the string and start slinging it around… and wonder why it’s so silent! Make sure they have a fat enough rubber band (about 3.5” x ¼ “ – or larger) or they won’t get a sound.


Variations include: multiple rubber bands, different sizes of rubber bands, and trying it without the index card attached. The Buzzing Hornet works because air zips past the rubber band, making it vibrate, and the sound gets amplified just a bit by the index card.


Exercises 


  1. What effect does changing the length of the string have on the pitch?
  2. What vibrates in this experiment to create sound?
  3. Why do we use an index card?

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Harmonicas

Monday July 27, 2009

Your voice is a vibration, and you can feel it when you place a hand on your throat when you speak. As long as there are molecules around, sound will be traveling though them by smacking into each other.


That’s why if you put an alarm clock inside a glass jar and remove the air, there’s no sound from the clock. There’s nothing to transfer the vibrational energy to – nothing to smack into to transfer the sound. It’s like trying to grab hold of fog – there’s nothing to hold on to.


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Here’s what you need:


  • two tongue depressors
  • three rubber bands, one at least 1/4″ wide
  • paper
  • tape


 
Download Student Worksheet & Exercises


What’s going on? The rubber band vibrates as you blow across the rubber band and you get a great sound. You can change the pitch by sliding the cuffs (this does take practice).


Troubleshooting: This project is really a variation on the Buzzing Hornets, but instead of using wind to vibrate the string, you use your breath. The rubber band still vibrates, and you can change the vibration (pitch) by moving the cuffs closer together or further apart. If the cuffs don’t slide easily, just loosen the rubber bands on the ends. You can also make additional harmonicas with different sizes of rubber bands, or even stack three harmonicas on top of each other to get unusual sounds.


If you can’t get a sound, you may have clamped down too hard on the ends. Release some of the pressure by untwisting the rubber bands on the ends and try again. Also – this one doesn’t work well if you spit too much – wet surfaces keep the rubber band from vibrating.


Exercises


  1. What is sound?
  2. What is energy?
  3. What is moving to make sound energy?

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BONUS! Nine Sonic Vibes Experiments to Mystify Your Kids

Tuesday June 30, 2009

Kazoo

Cut a piece of tissue paper the same length as a plastic comb (make sure the comb’s teeth are close together). Fold the tissue paper in half, wrapping it around the teeth of the comb.  Place it lightly between your lips and hummm… you’ll feel an odd vibrational effect on your lips as your kazoo makes a sound! You can try different papers, including waxed paper, parchment, tracing paper, and more!


Poppers

Cut the neck off a small balloon (balloons made for water bombs work well) and stretch it over the opening of a film canister. Pinch the drum head and pull up before you release – POP! You can change the pitch by adjusting the stretch of the drum head.
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Bobby Pin Strummer

Straighten three bobby pins. (A bobby pin, when straightened, has two different sides – a smooth side and a rippled side.) Wrap a rubber band tightly around the base of an empty tin can. Slip a clothespin under the rubber band, jaw-end first so it clamps onto the rim. Place three clothespins around the rim of the soup can equally spread apart (about 120 degrees apart). Clamp the rippled end of a bobby pin into each clothespin, so that your contraction now looks like a can with three legs. Strum each pin, one at a time. What happens if you clamp the pins at different heights?


What’s happening? Plucking the pins is just like plucking the string of a guitar, and when you change the heights, you’re changing the pitch. When the pin is shorter, it tends to vibrate faster, thus giving you a higher pitch.


String Test

Push the end of a length of string and a length of light thread through one hole punched in the bottom of a can. Tie the ends inside the can to a paperclip so they stay put. The can should have two different strings coming out of the bottom. Place the can near your ear as you strum each strand (hold the light taut while you pluck it – you may need an extra set of hands.) Can you make the pitch go both high and then low? What other types of string (yarn, thread, clothesline, heavy string, steel cable, fishing line, etc. ) can you use?


What’s going on? When you pluck the string, it starts vibrating (moving back and forth really fast). The vibration in the string starts the bottom of the cup vibrating, which starts the air inside the cup vibrating, too! The cup helps focus those vibrations (sounds) to your ear.


Styro-Phone

Make yourself an old-fashioned telephone by punching a small hole in the bottom of two cups (foam, paper, tin soup cans… is there a difference?) and threading string into each one. Tie the end of the string inside the cup to a paper clip so the string stays put. Does the string need to be tight, or does it work when its loose? How can you go around corners?


What’s going on? When you talk into the cup, you are making the air molecules bang around (vibrate), and some of them bang into the end with the string, which also picks up the vibration. The vibration continues along that string and into the receiver cup, which focuses the sound so you can hear it. The cup channels your voice into the other person’s ear.


Variation: Cut the phones apart and tie each end to a slinky and test it out (we call these “Space Phones”, and after you try it, you’ll see why). What happens when you bang the slinky into different things (like walls, metal chairs, wood tables, or the floor)?


Mystery Pitch

Blow across the mouth of an empty soda or water bottle to make a whistling sound. Add a little water and try again. Add more water and try again. Add more water. What happens if you use a glass bottle?


Place an empty glass under the sink faucet and tap the side of it with a fork and listen to the sound. Slowly fill the glass with water while you continue to tap. What happens if you use a spoon? Knife? Whisk? Wooden spoon?


Which of the experiments above (adding water to the bottle or removing water from the bottle) increases the pitch and which decreases the pitch?


Sonic Rulers

Hold one end of a ruler tightly on the table, overhanging half the length off the table. Pluck the free end and listen… (lift and let go… WHAP!) What if you make the free end of the ruler shorter? Longer? Wood? Plastic? Metal? Two rulers? Stacked? Side-by-side?


Sneaky Clocks

Place an alarm clock (the kind that ticks) or a timer that is ringing on a table and listen. Now place your ear on the table. Fill a zipper bag of water and press it between you and the clock to hear the difference. Next, place the clock in a closed metal can (like a cookie tin or coffee can). What about a paper bag? A glass jar? A newspaper-filled shoebox?


Shoebox Guitar

You can illustrate the vibrating string principle using a guitar string – when you pluck the string, your ears pick up a sound. If you have extra rubber bands, wrap them around an open shoebox to make a shoebox guitar.  You can also cut a hole in the lid (image left) and use wooden pencils to lift the rubber band off the surface of the shoebox.
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Ten Static Electricity Experiments to Mystify Your Kids

Saturday March 7, 2009

Plasma ball centerThe smallest thing around is the atom, which has three main parts – the core (nucleus) houses the protons and neutrons, and the electron zips around in a cloud around the nucleus.


The proton has a positive charge, and the electron has a negative charge. In the hydrogen atom, which has one proton and one electron, the charges are balanced. If you steal the electron, you now have an unbalanced, positively charge atom and stuff really starts to happen. The flow of electrons is called electricity. We’re going to move electrons around and have them stick, not flow, so we call this ‘static electricity’.


These next experiments rely heavily on the idea that like charges repel and opposites attract. Your kids need to remember that these activities are all influenced by electrons, which are very small, easy to move around, and are invisible to the eye.
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Blow up a balloon. If you rub a balloon on your head, the balloon steals the electrons from your head, and now has a negative charge. Your head now has a positive charge because your head was electrically balanced (same number of positive and negative charges) until the balloon stole your negative electrons, leaving you with an unbalanced positive charge.


Let’ play with a more static electricity experiments, including making things move, roll, spin, chime, light up, wiggle and more using  static electricity!


Here’s what you need:


  • 7-9″ balloon (get two in case one pops)
  • a wall
  • wool sweater or scarf
  • sink
  • ping pong ball
  • comb
  • neon bulb
  • tissue paper
  • wire coat hanger
  • tape
  • packing peanuts
  • bubble juice
  • fluorescent bulb (burnt-out bulbs are fine to use)
  • nylon stocking (AKA ‘panty-hose’)
  • plastic grocery bag

Download Student Worksheet & Exercises


Static Electricity Experiments!

Expt. 1: Static Hairdo Charge a balloon by rubbing it on your head for 30 seconds. Pull the balloon up about six inches to check your progress – if the hair isn’t sticking to the balloon, try again on someone with clean, dry hair (without any hair styling goop). When you put the balloon close to your head, notice how your hair reaches out for the balloon. Your hair is positive, the balloon is negative, and you can see how they are attracted to each other!
Your hair stands up when you rub it with a balloon because your head is now positively charged, and all those plus charges don’t like each other (repel). They are trying to get as far away from each other as possible, so they spread far apart. Does the hair continue to stand apart even after you remove the balloon? Does it matter what hair color or texture? (Does the balloon shape matter?)


Bonus Question: How can you get rid of the extra electrons?


Expt. 2: Finding Attraction Rub your head with the balloon and then hold the balloon to a wall. Can you make it stick? How about the ceiling? How many other things does the balloon stick to? (Hint – try a wool sweater.)


Expt. 3 Wiggly Wonder Hold the charged balloon near a stream of water running from a faucet. Can you make the water wiggle without touching it? The charged balloon attracts the stream of water. The water is like a bar magnet in that there are poles on a water molecule: there’s a plus side and a minus side, and the water molecules line up their positive ends toward the balloon when you bring it close.


Expt. 4 Ping Pong Puzzle Rub a comb with a wool sweater, and bring it close to a ping pong ball resting on a flat table. Why do you think the ping pong ball moves? Does it work if you use a charged balloon instead? What if you swap the ping pong ball for a piece of styrofoam?


Expt. 5: Static Neon Store up a good charge of electrons by scuffing along the carpet in socks on a warm, dry day. To make this a much more interesting experiment, hold one end of a neon bulb and watch it light as you touch the other end to a nearby object such as a metal faucet, metal part of a lamp, etc. You can also bring it close to your TV set (the old tube TV kind), both turned on and just turned off, to see if it has any effects on the neon lamp bulb?


Hint: you’ll need to get the neon bulb out of the plastic encasing and hold only one of the wires to make this experiment work – one wire act a as the collector, the other is grounded (via your hand) to the earth. You can also hold onto one lead as you slide down a plastic slide and then touch something grounded (like your mom).


You steal electrons and take on a negative charge when you scuff along the carpet in socks. Remember that just like magnets, ‘like’ charges (negative-to-negative or positive-to-positive) repel, and opposite charges (negative-to-positive) attar, which is why you can make your hair stand up on end by scuffing around a lot. The hairs all become negative, trying to get as far away from each other as they can.


Expt. 6: Electric Tail Feathers Cut a sheet of tissue paper into 12 thin strips, about 1/2″ wide and 8-12″ long. Straighten out a wire coat hanger (snip off the hook part), or find yourself a 10g piece of metal uninsulated wire. Tape the strips to the end of a wire coat hanger (make sure your coat hanger do not have plastic insulation around it – use sandpaper to sand off any clear enamel if you’re not sure). Attach a piece of plastic with tape or clay to the center of the rod, making a V-groove so the handle sits better on the wire. Bring a very charged balloon near the end of the wire – what happens?


Expt. 7: Ghost Words (Although this experiment has also held the name “Ghost Poop”… ) Rub packing peanuts with wool or your hair to build up a strong, quick static charge. Stick the stryfoam to the wall to spell out words. How long do they stay attached to the wall? Does humidity matter? (Try spritzing with a light mist of water).


Expt. 8: Static Bubbles Blow a few big, round bubbles (use store-bought bubble solution, or make your own with 12 cups cold water and 1 cup clear Ivory dish soap and a wire coat hanger stretched into a diamond shape). Chase your bubbles with a charged balloon – what happens? Try the comb rubbed with wool – which works better? What other two things can you use to change the path of the soap bubble? (Photo: Tom Noddy, one of the greatest bubble magicians ever – he’s the first one to ever blow a square bubble.)


Expt. 9: Fluorescents Unplugged In a dark room, rub the length of a fluorescent bulb with a piece of plastic wrap (or polyethylene bag or wool sweater) vigorously and then pull your arm away – the bulb should light up momentarily. What other materials cause it to glow?


Expt. 10: Ghost Leg This experiment is absolutely hilarious to watch, but you must be persistent to get it right. On a cold winter day, crank up the heat in your house to warm and dry out the air. You now have the ideal static electricity environment. Take a nylon stocking (just a single knee-length will work, or just use half of a full pair, but roll up the unused half so it’s out of your way) and press the toe part against a nearby wall. Line your other hand with a piece of a clear plastic bag (if the plastic can stretch, it’s the right kind) and rub the nylon stocking vigorously. Now hold the stocking in the air and see if you scrubbed it well enough to charge the stocking with enough static charge so it repels itself and fills out – looking as if there’s a ghost filling out the leg!


Why do these experiments work?

The triboelectric series is a list that ranks different materials according to how they lose or gain electrons. Near the top of the list are materials that take on a positive charge, such as air, human skin, glass, rabbit fur, human hair, wool, silk, and aluminum. Near the bottom of the list are materials that take on a negative charge, such as amber, rubber balloons, copper, brass, gold, cellophane tape, Teflon, and silicone rubber.


When you rub a glass rod with silk, the glass takes on a positive charge and the silk holds the negative charge. When you rub your head with a balloon, the hair takes on a positive charge and the balloon takes on a negative charge.


When you scuff along the carpet, you build up a static charge (of electrons). Your socks insulate you from the ground, and the electrons can’t cross your sock-barrier and zip back into the ground. When you touch someone (or something grounded, like a metal faucet), the electrons jump from you and complete the circuit, sending the electrons from you to them (or it).


The fluorescent bulb lights up when the electrons jump around. The inside of the bulb is coated with phosphor (a white powder) and filled with mercury vapor gas. The phosphor gives off light whenever it gets smacked with UV light. The mercury vapor gives off UV light whenever it gets excited by electricity (movement of electrons). When you rub the outside of the bulb, electrons start to jump around, exciting the gas, which generated UV light, which hits the phosphor and causes it to glow briefly. When the bulb is in balance, it stays dark. If you tip the balance, electrons flow and you get light.


Exercises


  1. Why does the hair stick to the balloon?
  2. How do you get rid of electrons?
  3. Can you see electrons? Why or why not?
  4. Does it matter what kind of hair you rub the balloon on?
  5. How long does the hair continue to stand up after you remove the balloon?
  6. Does it matter what kind of balloon you use?
  7. How fast or slow do you need to rub for the biggest charge on the balloon?
  8. Does hair color matter?
  9. This evening, find an article or story that describes how electricity improves our lives. Bring the article to school. If you bring in an article that no one else brings in, you get extra points.

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Density

Thursday February 11, 1019

Density can be found by weighing an object and dividing by the volume of the object, and for geologists, is the same thing as specific gravity. Water has a density of 1, which means that 1 gram of water takes up 1 cubic centimeter of space. Specific gravity is a number you get when you divide the density of an object by the density of water, which happens to be 1 gram/cm3.


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Materials


  • Measuring cup that has graduation marks for milliliters (mL)
  • Scale that measures in grams
  • Rock samples (in the video: quartz, meteorite, pumice)


Download worksheet and exercises


The specific gravity (also called the “s.g.” or “SG”) of a mineral or rock is how we compare the weight of the sample with the weight of an equal volume of water. Low specific gravity substances, like pumice (0.9), are not very dense. High specific gravity substances, like for gold (19.3), are very dense. If the specific gravity is less than 1, it will float on water.


Density is a way to measure two different minerals that might be exactly the same size, but their weights are different. Minerals with a metallic luster tend to be heavier. You’ll find variations for SG within the same minerals due to impurities of the mineral. Along those lines, this test can’t be done for material that is embedded within a rock, only for a single sample.


Here are a few examples for you to compare your samples with:


  • Amber 1.1
  • Quartz 1.5
  • Obsidian 2.5
  • Amethyst 2.6
  • Diamond 3.5
  • Hematite 5.05
  • Pyrite 5.1
  • Gold 19.3
  1. Label and number each of your samples and record this on your data table.
  2. Weigh each sample and record the information on your table.
  3. Fill your cup with water and note the level.
  4. Completely submerge your sample and read the new water height.
  5. Subtract #4 from #3 answers to get the amount of water your sample displaces. Record this in your data table.
  6. Find the volume of water displaced for every sample.
  7. Divide the mass of the object by the volume to find the density: ρ = m / V with the units of grams / mL
  8. Note: 1 mL = 1 cm3

Exercises


  1. In your data table, which number was the same for every trial run?
  2. What is the equation for finding density?
  3. How did you find the volume of the rock?

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