Air Resistance

Thursday October 23, 2014

Remember when we studied Free Fall Motion and we assumed that all objects fall with the same acceleration of g or 9.81 m/s2 ?


Well, that wasn’t the whole truth, because not all objects fall with the same acceleration. But it’s a good place to start out when we’re getting our feet wet with physics. (You’ll find this happens a lot when you get to more advanced concepts… you learn the easier stuff first by ignoring a lot of other things until you can learn how to incorporate more things into your equations.) So why do objects stop accelerating and reach terminal velocity, and how why do more massive objects fall faster than less massive? To answer this, we’ll take a look at air resistance and Newton’s Second Law using the F = ma equation.


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Click here to go to next lesson on Drag Force.

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Drag Force

Thursday October 23, 2014

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?

Click here to go to next lesson on Two-body problems.

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Two-body problems

Thursday October 23, 2014

There are situations where you have two objects interacting with each other, which means that you’ll have two unknown variables you’ll solve for (usually acceleration). You can solve these types of problems in a couple different ways. First, you can look at the entire system and consider both objects as only one object. For example, the Earth and Moon might be combined into one object if we’re looking at objects that orbit the sun, so the mass of the Earth and Moon would be combined into a single mass, m, and would also have the same acceleration, a. This approach is used if you really don’t care about what’s going on between the two objects. Or you could treat each object as it’s own separate body and draw FBD for each one. This second approach is usually used if you need to know the forces acting between the two objects.


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Click here to go to next lesson on Newton’s Third Law of Motion.

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Forces

Thursday October 23, 2014

If I asked you to define the word force, what would you say? You probably have a feeling for what force means, but you may have trouble putting it into words. It’s kind of like asking someone to define the word “and” or “the”. Well, this lesson is all about giving you a better feeling for what the word force means. We’ll be talking a lot about forces in many lessons to come. The simplest way to define force is to say that it means a push or a pull like pulling a wagon or pushing a car. That’s a correct definition, but there’s a lot more to what a force is than just that.


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Click here to go to next lesson on Four Fundamental Forces.

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Four Fundamental Forces

Thursday October 23, 2014

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:


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  • 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.

Click here to go to next lesson on Types of Forces.

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Types of Forces

Thursday October 23, 2014

There are two big categories that forces fall into: contact forces, and forces resulting from something called action-at-a-distance, like gravitational, magnetic, and electrical forces. Contact forces come into play when objects are physically touching each other, like friction, air resistance, tension, and applied forces (like when your hand pushes on something, or you kick a ball with your foot).


Action-at-a-distance forces show up when the sun and planets pull on each other gravitationally. The sun isn’t in contact with the Earth, but they still exert a force on each other. Two magnets repel each other even though they don’t touch… that’s another example of action-at-a-distance force. Inside an atom, the protons and the electrons pull on each other via the electrical force.


The units of force are in “Newtons”, or N like this: “my suitcase weighs 20N”. 1 N = 1 kg * m/s2. A force is also a vector, meaning that is has magnitude and direction. The force my suitcase exerts on the ground is 20N in the downward direction. Scientists and engineers use arrows to indicate the direction of force.


We’ll learn how to do this by drawing “Free Body Diagrams”, or FBDs. These are really useful for inventors and engineers, because with one look at a structure or machine, they can see all the forces acting on it and quickly be able to tell if the object is experiencing unbalanced forces, and if so what would happen. Unbalanced forces can cause rockets to crash, aircraft to somersault, bridges to collapse, trains to roll off the track, skyscrapers to topple, machines to explode or worse!


We’re going to learn how to see forces by making a model of the real world down on paper, drawing in all the forces acting on the object and use a little math to figure out important information like acceleration, force and velocity. Most engineers and scientists spend a year or more studying just this one concept about FBDs (and also MADs: “Mass-Acceleration Diagrams”) in college, so let’s get started…


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Click here to go to next lesson on Resultant Force.

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Resultant Force

Thursday October 23, 2014

In order to figure out what’s going on with an object, you have to take a look at the forces being applied to it. Forces are a vector, meaning that they have a direction and a magnitude. Your weight is not just a number, but it’s also in the downward direction. When we look at the forces that act on an object (or system of objects), we need to know how to combine all the forces into a single, resultant force which makes our math a lot easier. Here’s a set of videos that will show you how to do this:


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Click here to go to next lesson on Practicing Resultant Forces.

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Practicing Resultant Forces

Thursday October 23, 2014

Now let’s take it a step further and look at how you’d analyze a ball being yanked on by two kids in different directions:


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Click here to go to next lesson on Vector Notation & Trigonometry.

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Vector Notation & Trigonometry

Thursday October 23, 2014

There’s a different type of notation for x and y axes called “i-hat” and “j-hat”. This next video will show you exactly what you need to know to understand how to use them together so you don’t get confused! If you haven’t learned about “sines” or “cosines” yet, or it’s been awhile since you’ve studied triangles, this video will show you exactly what you need to know in order to solve physics problems. We’re not going to spend time deriving where these came from (if you’re interested in that, just open up a trigonometry textbook), but rather we’re going to learn how to use them in a way that real scientists and engineers do.


Take out your notebook and take notes on the law of sines, law of cosines, and write down definitions for sine, cosine, and tangent based on what you learn in the video, especially if you’re new to all this. Take it slow and you’ll catch on soon enough, because math isn’t just a shiny box of tools you just learn about, but you need to take the tools out of the box and learn how to crank with them. And sometimes, you learn how to use a impact driver when you need it, not ahead of time for someday when you might need it. Don’t get stuck if you haven’t seen some of these math principles yet or if they don’t make sense where they came from – just start using them and your brain will pick it up on the way as you learn how to apply them. Again, don’t feel like you have to complete a comprehensive course in trig to be able to figure out how to add vectors together! Just follow these simple steps…


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Click here to go to next lesson on Using Vectors and Trig in Physics Problems.

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Using Vectors and Trig in Physics Problems

Thursday October 23, 2014

Now let’s put the coordinate systems together with vector addition into this more realistic problem we’re going to run into with our study in physics:


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Click here to go to next lesson on Force fields.

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Force fields

Thursday October 23, 2014

Force fields aren’t just something for science fiction writers. They are actually a very real and very mysterious part of the world in which you live. So, what is a force field? Well, I can’t tell you. To be honest, nobody can. There’s quite a bit that is still unknown about how they work. A force field is a strange area that surrounds an object. That field can push or pull other objects that wander into its area. Force fields can be extremely tiny or larger than our solar system.


A way to picture a force field is to imagine an invisible bubble that surrounds a gizmo. If some other object enters that bubble, that object will be pushed or pulled by an invisible force that is caused by the gizmo. That’s pretty bizarre to think about isn’t it? However, it happens all the time. As you sit there right now, you are engulfed in at least two huge force fields, the Earth’s magnetic field and the Earth’s gravitational field.


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.


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Click here to go to next lesson on Gravitational Fields & Weightlessness.

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Gravitational Fields & Weightlessness

Thursday October 23, 2014

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.


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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!)


Click here to go to next lesson on Weight & Mass.

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Weight & Mass

Thursday October 23, 2014

The difference between weight and mass often trips up college students, so let’s straighten this out. The mass of an object is how much stuff something is made out of, and the weight is the force of gravity acting on it. Mass deals with how much stuff there is, and weight deals with the pull of the Earth. Mass will never change no matter where you put the object, unless you take a bite out of it or pile more stuff on top of it. The weight can change depending on where you place it, like on another planet.


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Click here to go to next lesson on Gravity on Other Planets.

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Gravity on Other Planets

Thursday October 23, 2014

If you could stand on the Sun without being roasted, how much would you weigh? The gravitational pull is different for different objects. Let’s find out which celestial object you’d crack the pavement on, and which your lightweight toes would have to be careful about jumping on in case you leapt off the planet.


Weight is nothing more than a measure of how much gravity is pulling on you. Mass is a measure of how much stuff you’re made out of. Weight can change depending on the gravitational field you are standing in. Mass can only change if you lose an arm.


Materials


  • Scale to weigh yourself
  • Calculator
  • Pencil

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Download Student Worksheet & Exercises


  1. We need to talk about the difference between weight and mass. In everyday language, weight and mass are used interchangeably, but scientists know better.
  2. Mass is how much stuff something is made out of. If you’re holding a bowling ball, you’ll notice that it’s hard to get started, and once it gets moving, it needs another push to get it to stop. If you leave the bowling ball on the floor, it stays put. Once you push it, it wants to stay moving. This “sluggishness” is called inertia. Mass is how much inertia an object has.
  3. Every object with mass also has a gravitational field, and is attracted to everything else that has mass. The amount of gravity something has depends on how far apart the objects are. When you step on a bathroom scale, you are reading your weight, or how much attraction is between you and the Earth.
  4. If you stepped on a scale in a spaceship that is parked from any planets, moons, black holes, or other objects, it would read zero. But is your mass zero? No way. You’re still made of the same stuff you were on Earth, so your mass is the same. But you’d have no weight.
  5. What is your weight on Earth? Let’s find out now.
  6. Step on the scale and read the number. Write it down.
  7. Now, what is your weight on the Moon? The correction factor is 0.17. So multiply your weight by 0.17 to find what the scale would read on the Moon.
  8. For example, if I weigh 100 pounds on Earth, then I’d weight only 17 pounds on the Moon. If the scale reads 10 kg on Earth, then it would read 1.7 kg on the Moon.

What’s Going On?

Weight is nothing more than a measure of how much gravity is pulling on you. This is why you can be “weightless” in space. You are still made of stuff, but there’s no gravity to pull on you so you have no weight. The larger a body is, the more gravitational pull (or in other words 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, the dog and I both have gravitational fields! Since we are both bodies of mass, we have a gravitational field which will pull things toward us. All bodies have a gravitational field. However, my mass is so 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 tens 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!)


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, let’s 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. So the bowling ball should accelerate faster since there’s more force on it. However, the bowling ball is heavier so 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 years, it still remains one of the most elusive mysteries of science. At this point, nobody knows what makes things move toward a body of mass.


Why did the rock drop toward 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!


Exercises


  1. Of the following objects, which ones are attracted to one another by gravity?
    a) Apple and Banana b) Beagle and Chihuahua c) Earth and You d) All of the above
  2. True or False: Gravity accelerates all things differently
  3.  True or False: Gravity pulls on all things differently
  4.  If I drop a golf ball and a golf cart at the same time from the same height, which hits the ground first?
  5.  There is a monkey hanging on the branch of a tree. A wildlife biologist wants to shoot a tranquilizer dart at the monkey to mark and study him. The biologist very carefully aims directly at the shoulder of the monkey and fires. However, the gun makes a loud enough noise that the monkey gets scared, lets go of the branch and falls directly downward. Does the dart hit where the biologist was aiming, or does it go higher or lower then he aimed? (This, by the way, is an old thought problem.)

Click here to go to next lesson on Force and Mass Units.

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Force and Mass Units

Thursday October 23, 2014

Units In the US system of units, both mass and weight are measured in “pounds” or “lb”. That’s a BIG problem, because mass isn’t the same as weight, so how could their units be the same? The answer is, the units are not the same, but they look very similar. The units for mass are kg (kilograms) or lbm (pronounced “pounds mass”) and the units for force are N (Newtons) or lbf (pronounced “pounds force”). The trouble comes in when we drop that third character and “lbf” or “lbm” becomes just plain “lb”. That’s the problem, and it’s a major headache for students to understand. Here’s the main thing I want you to remember: 1 lbm is NOT equal to 1 lbf. Here’s a video that will explain how you use both of these in a real world:


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Click here to go to next lesson on Friction.

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Friction

Thursday October 23, 2014

Friction is the force between two objects in contact with one another when one object moves (or tries to move) across another on the surface. Friction is dependent on the types materials that are in contact with one another (rubber versus leather, for example), and how much pressure is put on the materials, and whether the surfaces are wet, dry, hot, cold… it’s really complicated. Friction happens due to the electromagnetic forces between two objects. Friction is not necessarily due to the roughness of the objects but rather to chemical bonds “sticking and slipping” over one another.


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Click here to go to next lesson on Types of Friction.

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Types of Friction

Thursday October 23, 2014

Hovercraft 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 pressure inside of balloon. This air flows down through the nozzle and out the bottom, under the CD, lifting it slightly as it goes and creating a thin layer for the CD to float on.


Although this particular hovercraft only has a ‘hovering’ option, I’m sure you can quickly figure out how to add a ‘thruster’ to make it zoom down the table! (Hint – you will need to add a second balloon!)


Here’s what you need:


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  • 7-9″ balloon
  • water bottle with a sport-top (see video for a visual – you can also use the top from liquid dish soap)
  • old CD
  • paper cup (or index card)
  • thumbtack
  • hot glue gun
  • razor with adult help


There’s air surrounding us everywhere, all at the same pressure of 14.7 pounds per square inch (psi). You feel the same force on your skin whether you’re on the ceiling or the floor, under the bed or in the shower. An interesting thing happens when you change a pocket of air pressure – things start to move.


This difference in pressure causes movement that creates winds, tornadoes, airplanes to fly, and the air to rush out of a full balloon. An important thing to remember is that higher pressure always pushes stuff around. While lower pressure does not “pull,” we think of higher pressure as a “push”.


The stretchy balloon has a higher pressure inside than the surrounding air, and the air is allowed to escape out the nozzle which is attached to the water bottle cap through tiny holes (so the whole balloon doesn’t empty out all at once and flip over your hovercraft!) The steady stream of air flows under the CD and creates a cushion of air, raising the whole hovercraft up slightly… which makes the hovercraft easy to slide across a flat table.


Want to make an advanced model Hovercraft using wires, motors, and leftovers from lunch? Then click here.


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Advanced students: Download your Hovercraft Lab here.


Click here to go to next lesson on Static and Kinetic Friction.

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Static and Kinetic Friction

Thursday October 23, 2014

What if there’s a lot of friction? Have you ever felt that you need to give something a shove before it starts moving? You have to overcome static friction in order to experience kinetic friction. (Static friction is higher in magnitude than kinetic friction, generally speaking.)


The equation for determining the friction is: f = μ Fnormal, where μ = the coefficient of friction.


For kinetic friction: fkinetic = μk Fnormal, where μk = the coefficient of kinetic friction
For static friction: fstatic = μs Fnormal, where μs = the coefficient of kinetic friction


Scientists have to figure out μs and μk by doing experiments, and they compile that data in tables for others to look up when they need it. Here’s how you can do that very same experiment to determine the coefficient of friction between two surfaces:


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Click here to go to next lesson on Everyday Friction.

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Everyday Friction

Thursday October 23, 2014

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?

Click here to go to next lesson on Newton’s Second Law of Motion.

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Newton’s First Law of Motion

Thursday October 23, 2014

First Law of Motion: Objects in motion tend to stay in motion unless acted upon by an external force. Force is a push or a pull, like pulling a wagon or pushing a car. Gravity is a force that attracts things to one another. Gravity accelerates all things equally. Which means all things speed up the same amount as they fall.


Materials: ball
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What happens when you kick a soccer ball? The ‘kick’ is your external force. The ball will continue in a straight line as long as it can, until air drag, rolling resistance, and gravity cause it to stop.


Find out more about this key principle in Unit 1 and Unit 2.



Download Student Worksheet & Exercises


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)?

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Click here to go to next lesson on Newton’s Law of Motion in Detail.

Newton’s Law of Motion in Detail

Thursday October 23, 2014

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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Click here to go to next lesson on Inertia.

Inertia

Thursday October 23, 2014

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.

Click here to go to next lesson on Inertia in Real Life.

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Inertia in Real Life

Thursday October 23, 2014
Next time you watch a drag race, notice the wheels. Are they solid metal discs, or do they have holes drilled through the rims? I came up with this somewhat silly, but incredibly powerful quick science demonstration to show my 2nd year university students how one set of rims could really make a difference on the racetrack (with all other things being equal).

[am4show have='p8;p9;p12;p39;p92;' guest_error='Guest error message' user_error='User error message' ] Here's what you need: two unopened cans of soup.

One should be clam chowder, the other chicken broth. Prop up a long table up on one end about 6-12" (you can experiment with the height later). You're going to roll them both down the table at the same time. Which do you expect to reach to bottom first - the chicken or the clam?

Not only do my college students need to figure out which one will win, they also have to tell me why. The secret is in how you calculate the inertia of each. Take a guess, then watch the video, do the activity, then read the explanation at the bottom (in that order) to get the most out of this experiment.



Download Student Worksheet & Exercises

Inertia & History



Inertia is a quality of an object that determines how difficult it is to get that object to move, to stop moving, or to change directions. Generally, the heavier an object is, the more inertia it has. An elephant has more inertia than a mushroom. A sumo wrestler has more inertia than a baby. Inertia is made from the Latin word "inert," which means "lacking the ability to move". Inertia isn't something people have a grasp of, though, as it's something you must mathematically calculate from an object's mass and size.

When riding in a wagon that suddenly stops, you go flying out. Why? Because an object in motion tends to stay in motion unless acted upon by an outside force (Newton's First Law). When you hit the pavement, your motion is stopped by the sidewalk (external force). Seatbelts in a car are designed to keep you in place and counteract inertia if the car suddenly stops.

Did you know that Newton had help figuring out this First Law? Galileo rolled bronze balls down an wood ramp and recorded how far each rolled during a one-second interval to discover gravitational acceleration. And René Descartes (the great French philosopher) proposed three laws of nature, all of which Newton studied and use in his published work.

All of these thinkers (and many more) had to overcome the long-standing publicly-accepted theories that stemmed from the Greek philosopher Aristotle, which was no small feat in those days. Aristotle had completely rejected the idea of inertia (he also thought that weight affected falling objects, which we now know to be false). But remember that back then, people argued and talked about ideas rather than performing actual experiments to discover the truth about nature. They used words and reason to navigate through their world more than scientific experimentation.

Who wins, and why?

The chicken soup wins, for a very simple reason. Imagine that the cans are transparent, so you can see what does on inside the cans as they roll down the ramp. Which one has just the can rolling down the ramp, and which has the entire contents locked together as it rolls? The can of the chicken soup will rotate around the soup itself, while the clam chowder acts as a solid cylinder and rotates together. So the inertial mass of the clam is much greater than the inertial mass of the soup, even though the cans weigh the same.

Exercises
  1. What is inertia (in your own words)?
  2. Why does one soup can always win?

For Advanced Students...

[/am4show][am4show have='p9;p39;' guest_error='Guest error message' user_error='User error message' ] So, how do you calculate the inertia of the chicken soup and the clam? Here's the mathematical formulas from the back of a Dynamics textbook (a typical course that all Engineers take during their 2nd year of college).

Inertia of a solid cylinder = 1/2 * (mr²) Inertia of a cylindrical shell = 1/12 * (mr²)

If the radius of the soup is 6.5 cm and the mass for both is the same (345 grams, or 0.345kg), and the mass of an empty can is 45 grams, then:

(CLAM) Inertia of a solid cylinder = 1/2 * (mr²) = 1/2 * (0.345kg)*(6.5cm)² = 7.29 kg cm² (CHICKEN) Inertia of a cylindrical shell = 1/12 * (mr²) = 1/12 * (0.045kg) *(6.5cm)² = 0.158 kg cm²

The numerical value for the solid cylinder is larger than the shell, which tells us that it has a greater resistance to rolling and will start to rotate much slower than the shell. This makes logical sense, as it's easier to get the shell alone to rotate than move a solid cylinder. Remember, you must use the mass of the cylinder shell (empty can) when calculating the chicken's inertia, as the broth itself does not rotate and this does not have a 'rolling resistance'!

Advanced students: Download your Chicken and Clam Lab here. [/am4show]

Click here to go to next lesson on Introducing the Idea of Net Forces.


Introducing the Idea of Net Forces

Thursday October 23, 2014

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 wasn’t moving, but you had only one force moving down, what does that tell you about the force you and your friend exerted?

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Click here to go to next lesson on Forces.

Special Science Teleclass: Magnetism

Monday October 20, 2014

This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too!


Discover how to detect magnetic fields, learn about the Earth’s 8 magnetic poles, and uncover the mysterious link between electricity and magnetism that marks one of the biggest discoveries of all science…ever.


Materials:


  • Box of paperclips
  • Two magnets (make sure one of them ceramic because we’re going to break it)
  • Compass
  • Hammer
  • Nail
  • Sandpaper or nail file
  • D cell battery
  • Rubber band
  • Magnet Wire

Optional Materials if you want to make the Magnetic Rocket Ball Launcher:Four ½” (12mm) neodymium magnets


  • Nine ½” (12 mm) ball bearings
  • Toilet paper tube or paper towel tube
  • Ruler with groove down the middle
  • Eight strong rubber bands
  • Scissors

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Key Concepts

While the kids are playing with the experiments see if you can get them to notice these important ideas. When they can explain these concepts back to you (in their own words or with demonstrations), you’ll know that they’ve mastered the lesson.


Magnets


  • Magnetic fields are created by electrons moving in the same direction. Electrons can have a “left” or “right” spin. If an atom has more electrons spinning in one direction than in the other, that atom has a magnetic field.
  • If an object is filled with atoms that have an abundance of electrons spinning in the same direction, and if those atoms are lined up in the same direction, that object will have a magnetic force.
  • A field is an area around an electrical, magnetic or gravitational source that will create a force on another electrical, magnetic or gravitational source that comes within the reach of the field.
  • In fields, the closer something gets to the source of the field, the stronger the force of the field gets. This is called the inverse square law.
  • A magnetic field must come from a north pole of a magnet and go to a south pole of a magnet (or atoms that have turned to the magnetic field.)
  • All magnets have two poles. Magnets are called dipolar which means they have two poles. The two poles of a magnet are called north and south poles. The magnetic field comes from a north pole and goes to a south pole. Opposite poles will attract one another. Like poles will repel one another.
  • Iron and a few other types of atoms will turn to align themselves with the magnetic field. Over time iron atoms will align themselves with the force of the magnetic field.
  • The Earth has a huge magnetic field. The Earth has a weak magnetic force. The magnetic field comes from the moving electrons in the currents of the Earth’s molten core. The Earth has a north and a south magnetic pole which is different from the geographic north and south pole.
  • Compasses turn with the force of the magnetic field.

Electromagnetism


  • Electricity is moving electrons. Magnetism is caused by moving electrons. Electricity causes magnetism.
  • Magnetic fields can cause electricity.

What’s Going On?

The scientific principles we’re going to cover were first discovered by a host of scientists in the 19th century, each working on the ideas from each other, most prominently James Maxwell. This is one of the most exciting areas of science, because it includes one of the most important scientific discoveries of all time: how electricity and magnetism are connected. Before this discovery, people thought of electricity and magnetism as two separate things.  When scientists realized that not only were they linked together, but that one causes the other, that’s when the field of physics really took off.


Questions

When you’ve worked through most of the experiments ask your kids these questions and see how they do:


  1. How many poles do magnets have, and what are they?
  2. What happens when you bring two like poles together?
  3. How do I know which pole is which on a magnet?
  4. Is the magnetic force stronger or weaker the closer a magnet gets to another magnet?
  5. What kinds of materials are magnets made from?
  6. Name three objects that stick to a magnet.
  7. Name three that don’t stick to a magnet.
  8. What does a compass detect? How do you know when it’s detected it?

Answers:


  1. How many poles do magnets have, and what are they? Two. North and South poles.
  2. What happens when you bring two like poles together? They repel each other.
  3. How do I know which pole is which on a magnet? Put two magnets together and find the spot where they are repelling the strongest. The poles facing each other are the same. Or bring it close to a compass. If the magnet attracts the needle to north, then the magnet’s pole is the south pole.
  4. Is the magnetic force stronger or weaker the closer a magnet gets to another magnet? Stronger.
  5. What kinds of materials are magnets made from? Iron, nickel and cobalt.
  6. Name three objects that stick to a magnet. Paperclips, pipe cleaners, and staples.
  7. Name three that don’t stick to a magnet. US quarter, glass, plastic.
  8. What does a compass detect? How do you know when it’s detected it? The direction of a magnetic field. When the needle is deflected, the compass is in a magnetic field.

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Special Science Teleclass: Electricity

Monday October 13, 2014

This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too!


We’re going to study electrons and static charge. Kids will build simple electrostatic motor to help them understand how like charges repel and opposites attract. After you’ve completed this teleclass, be sure to hop on over the teleclass in Robotics!


Electrons are strange and unusual little fellows. Strange things happen when too many or too few of the little fellows get together. Some things may be attracted to other things or some things may push other things away. Occasionally you may see a spark of light and sound. The light and sound may be quite small or may be as large as a bolt of lightning. When electrons gather, strange things happen. Those strange things are static electricity.


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Materials:


  • Balloon (7-9″, inflated with air, not helium)
  • AA battery case
  • 2 AA batteries for your battery case (cheap dollar-store “heavy duty” type are perfect. Don’t use alkaline batteries if you can help it, because kids are going to short circuit their circuits, and the cheaper kind are safer in case they do.)
  • 1-2 LEDs
  • Alligator wires
  • 1.5-3V DC motor
  • 3-6V buzzer

If you want to make the laser burglar alarm, then get these also:


  • OPTIONAL: CdS Photoresistor for the laser burglar alarm
  • OPTIONAL: 9V Battery for laser burglar alarm
  • OPTIONAL: Laser pointer (the cheap kind from the dollar store work great) or strong flashlight for the laser burglar alarm

If you want to make the first robotics projects then also get these:


  • OPTIONAL: block of foam (any kind will do that is at least 2″ on each side)
  • OPTIONAL: 10 (or more) wood skewers at least 4″ long
  • OPTIONAL: 1 wood clothespin
  • OPTIONAL: Hot glue and glue sticks (with adult help)

If you want to make the second robotics project then also get these:


  • OPTIONAL: Additional 3V DV motor (you need two for this project)
  • OPTIONAL: 6 large popsicle sticks (tongue depressor size)
  • OPTIONAL: Tack or other sharp object for poking holes
  • OPTIONAL: Hot glue and glue sticks (with adult help)


Key Concepts

The proton has a positive charge, the neutron has no charge (neutron, neutral get it?) and the electron has a negative charge. These charges repel and attract one another kind of like magnets repel or attract. Like charges repel (push away) one another and unlike charges attract one another. Generally things are neutrally charged. They aren’t very positive or negative, rather have a balance of both.


Things get charged when electrons move. Electrons are negatively charged particles. So if an object has more electrons than it usually does, that object would have a negative charge. If an object has less electrons than protons (positive charges), it would have a positive charge. How do electrons move? It turns out that electrons can be kind of loosey goosey.


Depending on the type of atom they are a part of, they are quite willing to jump ship and go somewhere else. The way to get them to jump ship is to rub things together. Like in our experiment we’re about to do…


What’s Going On?

In static electricity, electrons are negatively charged and they can move from one object to another. This movement of electrons can create a positive charge (if something has too few electrons) or a negative charge (if something has too many electrons). It turns out that electrons will also move around inside an object without necessarily leaving the object. When this happens the object is said to have a temporary charge.


When you rub a balloon on your head, the balloon is now filled up with extra electrons, and now has a negative charge. Opposite charges attract right? So, is the entire yardstick now an opposite charge from the balloon? No. In fact, the yardstick is not charged at all. It is neutral. So why did the balloon attract it?


The balloon is negatively charged. It created a temporary positive charge when it got close to the yardstick. As the balloon gets closer to the yardstick, it repels the electrons in the yardstick. The negatively charged electrons in the yardstick are repelled from the negatively charged electrons in the balloon.


Since the electrons are repelled, what is left behind? Positive charges. The section of yardstick that has had its electrons repelled is now left positively charged. The negatively charged balloon will now be attracted to the positively charged yardstick. The yardstick is temporarily charged because once you move the balloon away, the electrons will go back to where they were and there will no longer be a charge on that part of the yardstick.


This is why plastic wrap, Styrofoam packing popcorn, and socks right out of the dryer stick to things. All those things have charges and can create temporary charges on things they get close to.


Questions to Ask

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

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1-D Kinematics

Tuesday October 7, 2014

We’re going to learn about kinematics, which is the words scientists use to explain the motion of objects. By learning about scalars, vectors, speed, velocity, acceleration, distance, and more, you’ll be able to not only accurately describe the motion of objects, but be able to predict their behavior. This is very important, whether you’re planning to land a spaceship on a moon, catapult a marshmallow in your mouth from across the room, or win a round of billiards.




Be sure to take out a notebook and copy down each example problem right along with me so you take good notes as you go along. It’s a totally different experience when you are actively involved by writing down and working through each problem rather than passively sitting back and watching.


Click here to start the first lesson in kinematics.


Free Fall and the Acceleration of Gravity

Friday October 3, 2014

If you jump out of an airplane, how fast would you fall? What’s the greatest speed you would reach? In a moment, we’re going to find out, but first let’s take a look at objects that are allowed to fall under the influence of just gravity. There are two important things to keep in mind for free falling objects. First, the object doesn’t experience air resistance. Second, the acceleration of the object is a constant value of 9.8 m/s2 or 32.2 ft/s2.
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Click here to go to next lesson on Acceleration.

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Acceleration

Friday October 3, 2014

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:


[am4show have=’p8;p9;p11;p38;p72;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • 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 fir

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Click here to go to next lesson on Velocity.

Velocity

Friday October 3, 2014

If acceleration is constant, is velocity also constant? Nope. The image at the top of this page shows that the object is speeding up every second by a certain rate, so velocity is not a constant value. The question is, can we figure out what the speed is at different intervals of time? Of course we can! Here’s how…
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Click here to go to next lesson on Graphing Free Fall Motion.

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Graphing Free Fall Motion

Friday October 3, 2014

Do you expect a curved or a straight line on a p-t graph for free falling objects? A straight line is the slope of the graph, which is also the velocity. A straight line would mean that the velocity is constant, we we already see from the experiment that it isn’t. So we can expect a curve on our p-t graph that looks like a downhill bunny slope… the object starts out slow, then increases speed so the slope will also increase in “steepness” as time goes on. If we indicate the positive direction as upwards, then the slope on the p-t graph will be negative.
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Click here to go to next lesson on Using the v-t Graph.

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Using the v-t Graph

Friday October 3, 2014

For constant acceleration, we can expect a straight line on our v-t graph to have a slope of 9.8 m/s2  in the negative quadrant of the graph, starting at the origin. The object started at rest, then finished with a large negative velocity, meaning that the object is speeding up in the downward direction. The constant negative slope means constant negative acceleration. Remember, that negative sign doesn’t mean it’s slowing down, but rather the minus sign indicates which direction the acceleration is happening in.
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Click here to go to next lesson on Gravity.

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Gravity

Friday October 3, 2014
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: [am4show have='p8;p9;p12;p39;p72;p92;' guest_error='Guest error message' user_error='User error message' ]
  • 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:

[/am4show] [am4show have='p9;p39;' guest_error='Guest error message' user_error='User error message' ] 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. [/am4show] 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?

[/am4show] 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]

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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.

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Click here to go to next lesson on Force and Mass.


More Hints on Solving Physics Problems

Friday October 3, 2014
We're going to learn more about why gravity accelerates all objects equally when we study Newton's Laws in the next section, where you'll discover how force is related to mass. Right now, here's another set of hints on solving physics problems...

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Yay! You've completed the first section! Now it's time for you to try solving these on your own:

Download your Practice Problem set here.

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Describing Motion with Equations

Thursday October 2, 2014
The motion of objects can be described by words, with images, as well as with the language of math by using graphs, charts, and equations.

We've already learned about the p-t and v-t graphs in our experiments, and now it's time to figure out the kinematic equations that will describe the motion of objects by relating the time, distance, displacement, velocity, speed, and acceleration. They're a really handy set of four equations that you can use to figure out how fast you're moving in a swing, how far your car will skid, the height your rocket will reach, or how far your baseball will go.

[am4show have='p9;p58;' guest_error='Guest error message' user_error='User error message' ]








The four Kinematic Equations are:

d = vi t + (1/2) a t2


vf2 = vi2 + 2ad


vf = vi + at


d = (1/2) (vi + vf) t


Where:

t = time
d = displacement
vf = final velocity
vi = initial velocity
a = acceleration

These equations describe the relationship between the variables (v, t, d, a) so that if you know three out of four of them, you can solve for the missing variable. The video below will show you how to use these equations in a number of sample problems so you can discover how to do this for yourself.



Click here to go to next lesson on Kinematic Equations Example Problems.


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Kinematic Equations Example Problems

Thursday October 2, 2014

Let’s try another example problem so you can see how to apply the equations to solve for things you really want to know…
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Click here to go to next lesson on Using Two Methods Together.

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Using Two Methods Together

Thursday October 2, 2014

Once you get the hang of how to solve the four kinematic equations, you can put this together with your understanding of the v-t and p-t diagrams to make a more complete picture of the motion in your system. Remember how we learned that the slope of the line on a v-t graph is the acceleration of the object, and that you could use the area bounded by the axis and the slope to find the displacement? Now you’ve got two ways of figuring out the displacement, velocity, acceleration, and time in any problem. How can you use the two methods together to make you more efficient and effective at solving physics programs? Here’s how…
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Click here to go to next lesson on Free Fall and the Acceleration of Gravity.

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Vector Diagrams

Wednesday October 1, 2014
Physics is your big chance to show off your inner artist by drawing what you see through a scientists eyes in a special way so others can understand your big ideas. We're going to practice making models in our mind of what's going on in the real world, and learning how to write it down on paper using the language of mathematics so you can communicate with others and work together designing your inventions and predicting what might happen next. All scientists, engineers, technicians, including folks like Feynman and Einstein, learned how to represent the real world on paper in a visual way using diagrams. (Although Nobel prize winner Dr. Richard Feynman got frustrated and invented his own diagrams, which we still use today in quantum mechanics.)

[am4show have='p9;p58;' guest_error='Guest error message' user_error='User error message' ] Physics is based in the real world, and we try really hard to make sense of what we can see, hear, detect, measure, and observe by using models to represent it. Vector diagrams show the direction and magnitude using an arrow, and are used to show velocity and acceleration in a diagram that engineers commonly use when analyzing the forces and motion of a system, like gears on a bike. The magnitude is shown by the size of the arrow, and the direction is indicated conveniently by the direction of the arrow.

Here is really fun experiment you can do to practice taking measurements and graphing vectors. You can make a simple rocket (video below), launch it a few times against a wall, mark the maximum height, and then use your measurements to make a vector diagram for both acceleration and velocity. Pick three to five different points along the path that the object traveled and draw the arrows to indicate the vector for velocity and acceleration. Your first point should be the starting point, your third at the midpoint, and your last point at the end of the trip.

Click here to go to next lesson on Graphing.

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Graphing

Wednesday October 1, 2014
Graphs are used all over the field of physics, and the p-t and v-t graphs are the ones used most for moving objects, especially when describing the projectile motion of objects. With one peek at the graph, you can tell a lot about what's going on, which is one reason they are so useful. You don't have to pour over pages of equations to get a sense of what's going on with the experiment.

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Click here to go to next lesson on The position-time "p-t" graph.

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The position-time “p-t” graph

Wednesday October 1, 2014
The position-time “p-t” graph is one that gets used a lot, and since it's axes are position and time, the slope of the line will give average velocity to describe the motion of an object.  If the velocity is constant, then the slope is constant and you'll see a straight line (either uphill or downhill). If velocity is changing, you'll see a curved rather than straight line for the slope. A steeper line indicates larger velocity. An uphill slope means positive velocity, downhill indicates negative velocity. If the slope is downhill and curved, but it starts out like a skier on a bunny hill, then the negative velocity starts slow and moves fast as time goes on, which is a sign of negative acceleration (starting slow and speeding up). If the slope looks instead like starting at the top of a black diamond run, then the object starts with a high negative velocity but ends with a slower velocity, a indication of positive acceleration.

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Click here to go to next lesson on The velocity-time "v-t" graphs.

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The velocity-time “v-t” graphs

Wednesday October 1, 2014
The velocity-time “v-t” graphs are another common type of graph you'll run across that describe motion of an object. The shape and slope of the lines on the graph will tell you a lot about what's going on with the motion of the object, and here's how you decipher it:  If the line is a straight, horizontal line, then the velocity stayed constant and there's no acceleration, like when you're driving on the freeway. Your car is moving at a steady 65 mph in a straight line.

However, if you're at a stoplight that just turned green, you're going to start changing your velocity by increasing your speed, giving you a positive acceleration. The graph will be a straight line starting at the origin and moving uphill. The slope of the line is positive, indicating your positive acceleration.

So can you tell if an object is moving in a positive or negative direction? Yes! A positive velocity means an object is moving in a positive direction, so if the line is in the positive region of the graph, you know it's traveling in a positive direction.  By the same logic, if the slope is in the negative regions of the graph, the object is traveling in a negative direction. For slopes crossing the axis, the object is changing directions.

Can you figure out if an object is speeding up or slowing down? Yes again! Speeding up means that the magnitude of the velocity is increasing in value (the number only, ignoring the plus or minus sign), so if the line is moving away from the x-axis, it's speeding up. And if it's approaching the x-axis, it's slowing down.

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Click here to go to next lesson on Slope of the Line.

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Slope of the Line

Wednesday October 1, 2014
soccerball1This experiment is one of my favorites in this acceleration series, because it clearly shows you what acceleration looks like. The materials you need is are:

  • a hard, smooth ball (a golf ball, racket ball, pool ball, soccer ball, etc.)

  • tape or chalk

  • a slightly sloping driveway (you can also use a board for a ramp that's propped up on one end)
For advanced students, you will also need: a timer or stopwatch, pencil, paper, measuring tape or yard stick, and this printout.

Grab a friend to help you out with this experiment - it's a lot easier with two people.

Are you ready to get started really discovering what acceleration is all about?

Here's what you do: [am4show have='p8;p9;p12;p39;p92;' guest_error='Guest error message' user_error='User error message' ] 1. Place the board on the books or whatever you use to make the board a slight ramp. You really don’t want it to be slanted very high. Only an inch or less would be fine. If you wish, you can increase the slant later just to play with it.

2. Put a line across the board where you will always start the ball. Some folks call this the “starting line.”

3. Start the timer and let the ball go from the starting line at the same exact time.

4. Now, this is the tricky part. When the timer hits one second, mark where the ball is at that point. Do this several times. It takes a while to get the hang of this. I find it easiest to have another person do the timing while I follow the ball with my finger. When the person says to stop, I stop my finger and mark the board at that point.

5. Do the exact same thing but this time, instead of marking the place where the ball is at one second, mark where it is at the end of two seconds.

6. Do it again but this time mark it at 3 seconds.

7. Continue marking until you run out of board or driveway.



Download Student Worksheet & Exercises Take a look at your marks. See how they get farther and farther apart as the ball continues to accelerate? Your ball was constantly increasing speed and as such, it was constantly accelerating. By the way, would it have mattered what the mass of the ball was that you used? No. Gravity accelerates all things equally. This fact is what Galileo was proving when he did this experiment. The the weight of the ball doesn’t matter but the size of the ball might. If you used a small ball and a large ball you would probably see differences due to friction and rotational inertia. The bigger the ball, the more slowly it begins rolling. The mass of the ball, however, does not matter.

Exercises

  1. Was the line a straight line?

  2. It should be close now, and the slope represents the acceleration it experienced going down the ramp. Calculate the slope of this line.

  3. What do you think would happen if you increased the height of the ramp?

  4. Knowing what you do about gravity, what is the highest acceleration it can reach?

For Advanced Students...

[/am4show][am4show have='p9;p39;' guest_error='Guest error message' user_error='User error message' ] Now if you want to whip out your calculators you can find out how fast your ball was accelerating. Take your measuring tape and measure the distance from the starting line to the line you made for the distance the ball traveled in one second.

Let’s say for example that my ball went 6 inches in that first second. Dust off those old formulas and lets play with d=1/2gt² where d is distance, g is acceleration due to gravity and t is time.

We can’t use g here because the object is not in free fall, so instead of g let’s call it “a” for acceleration. Gravity is the force pulling on our ball but due to the slope, the ball is falling at some acceleration less then 32 ft/s².

In this case, d is 6 inches, t is 1 second and a is our unknown.

With a little math we see:

a = 12in/sec² (So our acceleration for our ramp is 12 in/sec² or we could say 1ft/s².)

With a little more math we can see how far our ball should have traveled for each time trial that we did. For one second we see that our ball should have traveled d=1/2 12(12) or d= 6 inches (we knew that one already didn’t we?).

For two seconds we can expect to see that d=1/2 12(22) or d=24 inches. For three seconds we expect d=1/2 12(32) or d= 54 inches.

Do you see why we need a pretty long board for this?

Now roll the ball down the ramp and actually measure the distance it travels after two and three seconds. Do your calculations match your results? Probably not. Our nasty little friend friction has a sneaky way of messing up results. You should definitely see the distance the ball travels get greater with each second however. So make yourself a table or use one of ours to record your data and jot down your calculations and chart your results like a real scientist.

Advanced students: Download your Driveway Races Lab here.

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Click here to go to next lesson on Describing Motion with Equations


Special Science Teleclass: Black Holes

Monday September 29, 2014

This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too!


We’re ready to deal with the topic you’ve all been waiting for! Join me as we find out what happens to stars that wander too close, how black holes collide, how we can detect super-massive black holes in the centers of galaxies, and wrestle with question: what’s down there, inside a black hole?


Materials:


  • marble
  • metal ball (like a ball bearing) or a magnetic marble
  • strong magnet
  • small bouncy ball
  • tennis ball and/or basketball
  • two balloons
  • bowl
  • 10 pennies
  • saran wrap (or cup open a plastic shopping bag so it lays flat)
  • aluminum foil (you’ll need to wrap inflated balloons with the foil, so make sure you have plenty of foil)
  • scissors

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Key Concepts

What’s a black hole made of? Black holes are make of nothing but space and time, and they are the strangest things in nature.  It’s BLACK because does not emit or reflect light.  Black holes are the darkest black in the universe – no matter how powerful of a light you shine on it, even if it’s a million watt flashlight, no light ever bounces back, because its truly a ‘hole’ in space.


And a HOLE means nothing entering can escape. Anything that crosses the edge is swallowed forever. Scientists think of black holes as the edge of space, like a one-way exit door.


Biggest myth about black holes: Black holes are not vacuum cleaners with infinite sized bags. They do not roam around the universe sucking up everything they can find. They will grow gradually as stars and matter falls into them, but they do not seek out prey like predators. It just sits there with its mouth open, waiting for dinner.


Here’s an example of what a black hole is: If you take a ball and toss it up in the air, does it come back down to you? Sure! Toss it up even higher now… and it still comes back, right? What if you toss it up so fast that it exceeds the escape velocity of earth? (7 miles per second) Will it ever come back? No. The escape velocity depends on the gravitational pull of an object. The escape velocity of the sun is 400 miles per second. A black hole is an object that has an escape velocity greater than the speed of light. That’s exactly what a black hole is.


So, a black hole is a region where gravity is so strong that any light that tries to escape gets dragged back.  Because nothing can travel faster than light, everything else gets dragged back too!


Another interesting fact about black holes is that they are a place where gravity is so intense that time stops. This means that an object that falls into a black hole will never reappear, because they are frozen in time.


I often hear the question – how big are black holes? There’s no limit to the size of a black hole – it can be as large or as small as you can imagine it to be (and then some!). The more massive a black hole is, the more space it will take up, and the larger the radius of the event horizon. One of the largest and heaviest black holes is actually the super massive black hole at the center of our own Milky Way galaxy, about 30,000 light years away. Don’t worry, since it’s so far away and it’s not actively feeding.


Black holes are believed to be able to evaporate. Steven Hawking suggested that black holes aren’t exactly all black, but they emit a tiny bit of radiation, which comes directly from the black hole’s mass. This means as the black hole emits radiation, it loses mass, and shrinks.


If you’re looking for black holes, the nearest one is called V4641 Sgr and it’s 1,600 light years away in the Sagittarius arm of the Milky Way.  This is actually a rare type of black hole called a micro quasar. Click here for a downloadable Map of Black Holes.


One of the biggest misconceptions about black holes is that they are thought to be giant vacuum cleaners with infinitely large bags. Actually, they don’t go around vacuuming up all the matter they find. (If they did, they would eventually inhale all the matter in the universe and there’s be nothing left but black holes.) In fact, black holes can’t suck up all the matter because each black hole has its very own event horizon, which means that matter has to first cross that horizon in order to be eaten by the black hole. If it doesn’t go past that horizon, then it will not be sucked into the black hole.


Still crazy for black holes? Download the Exploring Black Holes PDF poster file and also try playing the Black Hole Space Travel game, which was developed by a team of NASA scientists. Enjoy!


Questions to Ask

  1. What are three different ways to detect a black hole?
  2. How many ways can a black hole kill you? Can you name them?
  3. What happens if you get close to a black hole, but not close enough to get sucked in? (Remember your magnet-marble experiment!)
  4. What if you watch someone get sucked in? What does it look like?
  5. What’s the most interesting thing you learned from the video about black holes?
  6. Why does a supernova explode? (Remember your two-ball experiment?)
  7. What causes a black hole to form?
  8. Does a black hole search for its next victim?
  9. Where is the closest super-massive black hole?
  10. What is gravitational lensing and why does it work? (Remember your marble-bowl experiment!)

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Introduction to Kinematics

Friday September 26, 2014

Mechanics is the study of the motion of objects. This is a great place to start your studies in physics since it’s such a BIG idea. We’ll be learning the language, laws, concepts, and principles that explain the motion of objects. We’re going to learn about kinematics, which is the words scientists use to explain the motion of objects. By learning about scalars, vectors, speed, velocity, acceleration, distance, and more, you’ll be able to not only accurately describe the motion of objects, but be able to predict their behavior. This is very important, whether you’re planning to land a spaceship on a moon, catapult a marshmallow in your mouth from across the room, or win a round of billiards.


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Click here to go to next lesson on Scalars and Vectors.


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