If you put an ice cube in a glass of lemonade, the ice cube melts. The thermal energy from your lemonade moves to the ice cube. Increasing the temperature of the ice cube and decreasing the temperature of your lemonade. The movement of thermal energy is called heat. The ice cube receives heat from your lemonade. Your lemonade gives heat to the ice cube. Heat can only move from an object of higher temperature to an object of lower temperature.
We’re going to learn about temperature, heat energy, atoms, matter, phase changes, and more in our unit on Thermodynamics as we build steam boats, fire-water balloons, hero engines, thermostats, Stirling engines, and more!
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NEW! Download the complete packet for this section here.
Does this sound familiar? “I’m too cold. Get me a sweater!” “This soup’s too hot!” “Phew, I’m sweating.” “Yowtch, that pan handle burned me!” If you’ve ever made any of the above comments, then you were talking about thermal energy. Very clever of you, don’t you think? Thermal energy is basically the energy of the molecules moving inside something. The faster the molecules are moving, the more thermal energy that something has. The slower they are moving, the less thermal energy that something has.
I’m sure at some point you’ve said, “Wow, my internal thermal energy is way high! I need a liquid with a low thermal energy.” What... you’ve never said that?! Oh, wait. I bet it sounded like this when you said it, “Wow, I’m hot! I need a cool drink.”
Whenever we talk about the temperature of something we are talking about its thermal energy. Objects whose molecules are moving very quickly are said to have high thermal energy or high temperature. The higher the temperature, the faster the molecules are moving. You may remember that temperature is just a speedometer for molecules.
You may have asked yourself the question, “So, if everything is made of molecules, and these molecules are often speeding up and slowing down…what happens to the stuff these molecules are are made of if they change speed a lot? Will my kitchen table start vibrating across the room if the table somehow gets too hot?” No, it’s pretty unlikely that your table will begin jumping around the room, no matter how hot it gets. However, some interesting things do happen when molecules change speeds.
Click here to go to next lesson on Thermal Physics Introduction.
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Here’s a teleclass to get you started on learning about Thermodynamics.
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The next set of lessons will take you in more detail on each topic covered in the teleclass (and more)!
Click here to go to next lesson on Temperature.
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Temperature is a way of talking about, measuring, and comparing the thermal energy of objects. We use three different kinds of scales to measure temperature. Fahrenheit, Celsius, and Kelvin. (The fourth, Rankine, which is the absolute scale for Fahrenheit, is the one you’ll learn about in college.)
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Mr. Fahrenheit, way back when (18th century) created a scale using a mercury thermometer to measure temperature. He marked 0° as the temperature ice melts in a tub of salt. (Ice melts at lower temperatures when it sits in salt. This is why we salt our driveways to get rid of ice). To standardize the higher point of his scale, he used the body temperature of his wife, 96°.
As you can tell, this wasn’t the most precise or useful measuring device. I can just imagine Mr. Fahrenheit, “Hmmm, something cold…something cold. I got it! Ice in salt. Good, okay there’s zero, excellent. Now, for something hot. Ummm, my wife! She always feels warm. Perfect, 96°. ” I hope he never tried to make a thermometer when she had a fever. Just kidding, I’m sure he was very precise and careful, but it does seem kind of weird.
Over time, the scale was made more precise and today body temperature is usually around 98.6°F. Later, (still 18th century) Mr. Celsius came along and created his scale. He decided that he was going to use water as his standard. He chose the temperature that water freezes at as his 0° mark. He chose the temperature that water boils at as his 100° mark. From there, he put in 100 evenly spaced lines and a thermometer was born.
Last but not least Mr. Kelvin came along and wanted to create another scale. He said, I want my zero to be ZERO! So he chose absolute zero to be the zero on his scale.
Click here to go to next lesson on Absolute Zero.
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Absolute zero is the theoretical temperature where molecules and atoms stop moving. They do not vibrate, jiggle or anything at absolute zero. In Celsius, absolute zero is -273 ° C. In Fahrenheit, absolute zero is -459°F (or 0°R). It doesn’t get colder than that!
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Click here to go to next lesson on Thermometers.
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As you can see, creating the temperature scales was really rather arbitrary: “I think 0° is when water freezes with salt.” “I think it’s just when water freezes.” “Oh, yea, well I think it’s when atoms stop!” Many of our measuring systems started rather arbitrarily and then, due to standardization over time, became the systems we use today. So that’s how temperature is measured, but what is temperature measuring?
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Click here to go to next lesson on Thermal Energy.
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Temperature is measuring thermal energy which is how fast the molecules in something are vibrating and moving. The higher the temperature something has, the faster the molecules are moving. Water at 34°F has molecules moving much more slowly than water at 150°F. Temperature is really a molecular speedometer. When something feels hot to you, the molecules in that something are moving very fast. When something feels cool to you, the molecules in that object aren’t moving quite so fast.
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Click here to go to next lesson on The Human Body.
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Have you ever wondered how an ice-cold glass of water gets waterdrops on the outside of the cup? Where does that water come from? Does it ease it’s way through the glass? Did someone come by and squirt the glass with water? No of course not.
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Some of the gaseous water molecules in the air came close enough to the cold glass to lose some molecular speed. Since they lost speed, they formed bonds between each other and liquefied. They condensed on the cold surface of the glass.
Imagine though, if you will, that you live several hundred years ago and the process of condensation wasn’t understood. You happen to be an inquisitive, highly perceptive, person (which of course you are) and you notice this film of water showing up on cold things. Water appearing out of apparently nowhere! You’d be pretty amazed wouldn’t you?!?
Personally, I still find it amazing that every time I pick up a cold can of soda there are molecular interactions happening right in front of my eyes! This is why science is so wonderful. It provides the skills to see these amazing things and the skills to investigate and perhaps understand them.
Crazy Temperatures
Here’s an experiment you can do to baffle your senses: Fill one glass with hot water (not boiling), another with ice water, and a third with room temperature water. Place a finger in the hot water and a finger in the ice cold water for a minute or two. Then stick both fingers in the room temperature cup.
How does that feel?
Did it seem that each finger detected a different temperature when placed in the room temperature cup? Weird! So what gives?
Materials:
- 3 cups of water (see video)
- your hands
Download Student Worksheet & Exercises
Your skin contains temperature sensors that work by detecting the direction heat flow (in or out of your body), not temperature directly. These sensors change temperature depending on their surroundings. So when you heated up one finger, and then placed it in cooler water, the heat flowed out of your body, telling your brain it was getting cooler. The ice water finger was detecting a heat flow into your body… and presto! You have one confused brain.
In order for heat to flow, you need to have a temperature difference. But why then do the metal legs of a table feel colder than the wood tabletop when both are at the same room temperature? The metal will feel colder because heat flows away from your skin faster into the metal than the wood. We’ll talk about heat capacity in a later experiment, but this is why scientists had to invent the thermometer, because the human body isn’t designed to detect temperature, only heat flow.
Click here to go to next lesson on Changing Molecular Speeds.
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You may have asked yourself the question, “So, if everything is made of molecules, and these molecules are often speeding up and slowing down… what happens to the stuff these molecules are made of if they change speed a lot? Will my kitchen table start vibrating across the room if the table somehow gets too hot?” No, it’s pretty unlikely that your table will begin jumping around the room, no matter how hot it gets. However, some interesting things do happen when molecules change speeds.
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Click here to go to next lesson on States of Matter.
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Is it warmer upstairs or downstairs? If you’re thinking warm air rises, then it’s got to be upstairs, right? If you’ve ever stood on a ladder inside your house and compared it to the temperature under the table, you’ve probably felt a difference.
So why is it cold on the mountain and warm in the valley? Leave it to a science teacher to throw in a wrench just when you think you’ve got it figured out. Let’s take a look at whether hot air or cold air takes up more space. Here’s what you do:
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Materials:
- water
- plastic bottle
- balloon
- stove top and saucepan or the setup in the video
Download Student Worksheet & Exercises
1. Pour a couple of inches of water into an empty soda bottle and cap with a 7-9″ balloon. You can secure the balloon to the bottle mouth with a strip of tape if you want, but it usually seals tight with just the balloon itself.
2. Fill a saucepan with an inch or two of water, and add your bottle. Heat the saucepan over the stove with adult help, keeping a close eye on it. Turn off the heat when your balloon starts to inflate. Since water has a high heat capacity, the water will heat before the bottle melts. (Don’t believe me? Try the Fire-Water Balloon Experiment first to see how water conducts heat away from the bottle!)
3. When you’re finished, stick the whole thing in the freezer for an hour. What happened to the balloon?
What’s going on? So now what do you think? Does the same chunk of air take up more or less space when it’s hot? Cold? When you heat air, it expands because the air molecules move around a lot more when you add energy. The molecules zip around and wiggle like crazy, bouncing off each other more often than when they are cooler.
Getting back to our original question, the upstairs in a house is warmer because the pockets of warm air rise because they are less dense than cool air. The more the molecules move around, the more room they need, and the further they get spaced out. Think of a swimming pool and a piece of aluminum foil. If you place a sheet of foil in the pool, it floats. If you take the foil and crumple it up, it sinks. The more compactly you squish the molecules together, the more dense it becomes. You can read more about density in Unit 3.
As for why mountains and valleys are opposite, it has to do with the Earth being a big massive ball of warm rock which heats up the lower atmosphere in addition to winds blowing on mountains and changes in pressure as you gain altitude… in a nutshell, it’s complicated! What’s important to remember is that the Earth system is a lot bigger than our bottle-saucepan experiment, and can’t be represented in this way.
Exercises
- Draw a group of molecules at a very cold temperature in the space below. Use circles to represent each molecule.
- True or False: A molecule that heats up will move faster.
- True
- False
- True or False: A material will be less dense at lower temperatures.
- True
- False
Click here to go to next lesson on Melting and Evaporation.
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Indoor Rain Clouds
Making indoor rain clouds demonstrates the idea of temperature, the measure of how hot or cold something is. Here’s how to do it:
Take two clear glasses that fit snugly together when stacked. (Cylindrical glasses with straight sides work well.)
Fill one glass half-full with ice water and the other half-full with very hot water (definitely an adult job – and take care not to shatter the glass with the hot water!). Be sure to leave enough air space for the clouds to form in the hot glass.
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Place the cold glass directly on top of the hot glass and wait several minutes. If the seal holds between the glasses, a rain cloud will form just below the bottom of the cold glass, and it actually rains inside the glass! (You can use a damp towel around the rim to help make a better seal if needed.)
Materials:
- glass of ice water
- glass of hot water (see video)
- towel
- adult help
Download Student Worksheet & Exercises
Bottling Clouds
On a stormy, rainy afternoon, try bottling clouds — using the refrigerator! Here’s what you do: Place an empty, clean 2-liter soda bottle in the fridge overnight. Take it out and get an adult to light a match, letting it burn for a few seconds, then drop it into the bottle. Immediately cap the bottle and watch what happens (you should see smoke first, then clouds forming inside). Squeeze the sides of the bottle. The clouds should disappear. When you release the bottle, the clouds should reappear. Materials:
- 2L soda bottle
- rubbing alcohol
- bicycle pump
- car tire valve (drill a 1/2 inch hole through a 2L soda bottle cap and pull the valve gently through with pliers)
Advanced Idea: You can substitute rubbing alcohol and a bicycle pump for the matches to make a more solid-looking cloud. Swirl a bit of rubbing alcohol around inside the bottle, just enough to coat the insides, and then pour it out. Cap your bottle with a rubber stopper fitted with a needle valve (so the valve is poking out of the bottle), and apply your pump. Increase the pressure inside the bottle (keep a firm hand on the stopper or you’ll wind up firing it at someone) with a few strokes and pull out the stopper quickly. You should see a cloud form inside.
What’s going on? Invisible water vapor is all around us, all the time, but they normally don’t stick together. When you squeezed the sides of the bottle, you increased the pressure and squeezed the molecules together. Releasing the bottle decreases the pressure, which causes the temperature to drop. When it cools inside, the water molecules stick to the smoke molecules, making a visible cloud inside your bottle.
Did you know that most drops of water actually form around a dust particle? Up in the sky, clouds come together when water vapor condenses into liquid water drops or ice crystals. The clouds form when warm air rises and the pressure is reduced (as you go up in altitude). The clouds form at the spot where the temperature drops below the dew point.
The alcohol works better than the water because it evaporates faster than water does, which means it moves from liquid to vapor more easily (and vividly) than regular old water.
Questions to ask:
- How many times can you repeat this?
- Does it matter what size the bottle is?
- What if you don’t chill the bottle?
- What if you freeze the bottle instead?
Exercises
- Which combination made it rain the best? Why did this work?
- Draw your experimental diagram, labeling the different components:
- Add in labels for the different phases of matter. Can you identify all three states of matter in your experiment?
Click here to go to next lesson on Condensation and Freezing.
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This spooky idea takes almost no time, requires a dime and a bottle, and has the potential for creating quite a stir in your next magic show. The idea is basically this: when you place a coin on a bottle, it starts dancing around. But there’s more to this trick than meets the scientist’s eye.
Here’s how you do it:
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Materials:
- coin
- freezer
- plastic bottle (NOT glass)
Download Student Worksheet & Exercises
Remove the cap of an empty plastic water or soda bottle and replace it with a dime and stick the whole things upright in the freezer overnight. First thing in the morning, take it out and set it on the table. What happens?
Attention: Magicians
If you’re a budding magician, here’s how to modify this experiment to use in your next show. Get a glass container of soda and stick it in the coldest part of your fridge overnight. I know they are getting harder to find these days, but the glass will keep its temperature longer than plastic and enable you to do this trick. In a pinch, you can refill a cleaned glass bottle from a previous use if you can’t locate a fresh glass soda bottle.
As soon as you’re ready to do your trick (practice first!), take it out of the fridge (have it in an ice chest if you’re on stage) and chug the whole thing. (You can ask your audience for help on this – you just want an empty cold bottle for the next part.)
When the bottle is empty but still cold, cap it with a wet dime. Place both hands on the bottle while you “wax eloquent” (make up an engaging story, like “North Winds, come have a taste of soda…”) but be sure to keep your hands on the bottle to warm it up. In about 20 seconds or so, the dime will click up and down, dancing around mysteriously. Keep your hands on the bottle for another 20 seconds or so, and then set it gently on the table with it still dancing, and you’ll find it dancing right on without your hands being there.
How does that work? Was the bottle really empty when you placed it in the freezer? Actually, no… it had air inside of it. The air in the bottle shrank down a bit as it cooled, allowing more air to go into the bottle. When you remove the bottle from the freezer and cap it with the coin, you now have a bottle with more air in it than you started. The air warms and expands, pushing the excess air out the top, making the coin dance. Learn more about air with this Air Expands experiment.
Variations to try:
- Is a plastic bottle or glass bottle better for this experiment?
- What if you get the coin wet first?
- Does a cold or warm coin work better?
- What about a penny? Quarter?
- Does the size of the bottle matter?
Exercises
- When a gas turns into a liquid, this is called:
- Convection
- Conduction
- Absorption
- Condensation
- When water boils, what happens to the bonds between its molecules?
- What is the best way to describe how the bonds between water molecules behave when in a liquid state?
- Solid bridges
- Rubber bands
- No bonds
- Brittle like chalk
- The crystalline shape of a solid is referred to as:
- a matrix
- a vortex
- a crystal
- a cube
Click here to go to next lesson on What are clouds?
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Clouds are made of hundreds of billions of tiny little droplets of liquid water that have condensed onto particles of some sort of dust. Now let’s turn the heat down a bit more and see what happens. As the temperature drops and the molecules continue to slow, the bonds between the molecules can pull them together tighter and tighter.
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Eventually the molecules will fall into a matrix, a pattern, and stick together quite tightly. This would be the solid state. The act of changing from a liquid to a solid is called freezing and the temperature at which it changes is called (say it with me now) freezing point.
Think about this for a second – is the freezing point and melting point of an object at the same temperature? Does something go from solid to liquid or from liquid to solid at the same temperature? If you said yes, you’re right! The freezing point of water and the melting point of water are both 32° F or 0° C. The temperature is the same. It just depends on whether it is getting hotter or colder as to whether the water is freezing or melting. The boiling and condensation point is also the same point.
Now I’m going to mess things up a little bit. Substances can change state at temperatures other than their different freezing or boiling points. Many liquids change from liquid to gas and from gas to liquid relatively easily at room temperatures. And, believe it or not, solids can change to liquids and even gases and vice versa at temperatures other than the usual melting, freezing, or boiling points. So what’s the point of the points?
Click here to go to next lesson on Changing States at Unusual Places.
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At a substance’s boiling, freezing, etc, points, all of the substance must change to the next state. The condition of the bonds cannot remain the same at that temperature. For example, at 100° C water must change from a liquid to a gas. That is the speed limit of liquid water molecules. At 100° C the liquid bonds can no longer hold on and all the molecules convert to gas.
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Click here to go to next lesson on Heat.
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Believe it or not, the concept of heat is really a bit tricky. What we call heat in common language, is really not what heat is as far as physics goes.
Heat, in a way, doesn’t exist. Nothing has heat. Things can have a temperature. They can have a thermal energy but they can’t have heat. Heat is really the transfer of thermal energy. Or, in other words, the movement of thermal energy from one object to another.
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If you put an ice cube in a glass of lemonade, the ice cube melts. The thermal energy from your lemonade moves to the ice cube. Increasing the temperature of the ice cube and decreasing the temperature of your lemonade. The movement of thermal energy is called heat. The ice cube receives heat from your lemonade. Your lemonade gives heat to the ice cube. Heat can only move from an object of higher temperature to an object of lower temperature.
Click here to go to next lesson on Heat goes from hot to cold.
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If you’ve completed the Soaking Up Rays experiment, you might still be a bit baffled as to why there’s a difference between black and white. Here’s a great way to actually “see” radiation by using liquid crystal thermal sheets.
You’ll need to find a liquid crystal sheet that has a temperature range near body temperature (so it changes color when you warm it with your hands.)
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The liquid crystal sheet is temperature-sensitive. When the sheet received heat from the bulb, the temperature goes up and changes color. The plastic sheets remain black except for the temperature range in which they display a series of colors that reflect the actual temperature of the crystal.
Materials:
- hands
- Thermal paper (AKA: Liquid Crystal Sheet)
- Incandescent light bulb (or sunny day)
- Silver highlighter or aluminum foil
1. Color half of the back side of the thermal paper (the side that doesn’t change color) with the highlighter (or cover half of it with foil).
2. Hold it in a position where you can easily see the color-changing side while keeping the light source on the back side.
3. Which side changes color? Is there a difference between the silver and black halves?
You’ll notice that the black half almost immediately changes color, while the silver side stays black. The silver coating reflects the heat, keeping it cool. The black side absorbs the heat and raises its temperature.
Why do liquid crystals change color with temperature? Your liquid crystal sheet is not just one sheet, but a stack of several sheets that are slightly offset from each other. The distance between each layer changes as the sheet warms up – the hotter the temperature, the closer the stacks twist together. The color they emit depends on the distance between the sheets.
The molecules that make up the sheets are long and thin, like hot dogs. When the sheets are cooler, these molecules move around less and don’t twist up as much, which corresponds to reflecting back a redder light. When the temperature rises, the molecules move around more and twist together, and they reflect a bluer light. When the liquid crystal sheet is black, all the light is absorbed (no light gets reflected).
Click here to go to next lesson on Heat Capacity.
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Now let’s take explore how, even though heat can move from one object to another, it doesn’t necessarily mean that the temperature of the objects will change. You may ask, “What? Heat can move from one object to another without temperature changing one little bit?!?!” We’re going to take a look at one of the ways heat can move while the thermometer doesn’t.
When things change phase (change from solid to liquid or liquid to gas or… well, you get the picture) the temperature of those objects don’t change. If you were able to take the temperature of water as it changed from a solid (ice) to a liquid you would notice that the temperature of that piece of ice will stay at about 32° F until that piece of ice was completely melted. The temperature would not increase at all. Even if that ice was in an oven, the temperature would stay the same. Once all the solid ice had disappeared, then you would see the temperature of the puddle of water increase.
By the way, as the ice is melting, from where is heat being transferred? Heat is being transferred, by conduction, from the air.
One key distinction is that objects don’t contain heat, but they contain energy. Heat is the transfer of energy from from one object to another, or from one system to another, like a hot cup of coffee to the cool ambient air. Heat can change the temperature of objects when it transfers the energy. In the example with the coffee cup, it lowered the temperature of the coffee.
Imagine putting a sponge under a slowly running faucet. The sponge would continue to fill with water until it reached a certain point and then water started to drip from it. You could say that the sponge had a water capacity. It could hold so much water before it couldn’t hold any more and the water started dripping out.
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Heat capacity is how much heat an object can absorb before it increases in temperature. It’s often used interchangeably with “specific heat capacity”, but in reality it’s a little different.
Click here to go to next lesson on Specific Heat Capacity.
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Specific heat capacity is how much heat energy a mass of a material must absorb before it increases 1°C. It’s how much heat is needed to raise the temperature of 1 gram of the material. Heat Capacity is how much heat is required to raise the temperature. The units of heat capacity are J per Kelvin, whereas for specific heat capacity, the units are J per (gram-K).
Each material has its own specific heat. The higher a material’s specific heat, the more heat it must absorb before it increases in temperature. Water is unique in that it has a very large specific heat. Liquid water’s specific heat is over 4 which is very high. In comparison, granite is 0.8, aluminum is 0.9, rubbing alcohol is 2.4 and gold is 0.1.
To get the same amount of rubbing alcohol and liquid water to increase the same amount of temperature, you would need to pump about twice the amount of heat into the water. To get the same amount of gold and liquid water to increase the same amount of temperature, you would need to pump 40 times the amount of heat into the water!
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Click here to go to next lesson on Fire Water Balloon.
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If you’ve ever had a shot, you know how cold your arm feels when the nurse swipes it with a pad of alcohol. What happened there? Well, alcohol is a liquid with a fairly low boiling point. In other words, it goes from liquid to gas at a fairly low temperature. The heat from your body is more then enough to make the alcohol evaporate.
As the alcohol went from liquid to gas it sucked heat out of your body. For things to evaporate, they must suck in heat from their surroundings to change state. As the alcohol evaporated you felt cold where the alcohol was. This is because the alcohol was sucking the heat energy out of that part of your body (heat was being transferred by conduction) and causing that part of your body to decrease in temperature.
As things condense (go from gas to liquid state) the opposite happens. Things release heat as they change to a liquid state. The water gas that condenses on your mirror actually increases the temperature of that mirror. This is why steam can be quite dangerous. Not only is it hot to begin with, but if it condenses on your skin it releases even more heat which can give you severe burns. Objects absorb heat when they melt and evaporate/boil. Objects release heat when they freeze and condense.
Do you remember when I said that heat and temperature are two different things? Heat is energy – it is thermal energy. It can be transferred from one object to another by conduction, convection, and radiation. We’re now going to explore heat capacity and specific heat. Here’s what you do:
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You Need:
- Balloon
- Water
- Matches, candle, and adult help
- Sink
Download Student Worksheet & Exercises
1. Put the balloon under the faucet and fill the balloon with some water.
2. Now blow up the balloon and tie it, leaving the water in the balloon. You should have an inflated balloon with a tablespoon or two of water at the bottom of it.
3. Carefully light the match or candle and hold it under the part of the balloon where there is water.
4. Feel free to hold it there for a couple of seconds. You might want to do this over a sink or outside just in case!
So why didn’t the balloon pop? The water absorbed the heat! The water actually absorbed the heat coming from the match so that the rubber of the balloon couldn’t heat up enough to melt and pop the balloon. Water is very good at absorbing heat without increasing in temperature which is why it is used in car radiators and nuclear power plants. Whenever someone wants to keep something from getting too hot, they will often use water to absorb the heat.
Think of a dry sponge. Now imagine putting that sponge under a slowly running faucet. The sponge would continue to fill with water until it reached a certain point and then water started to drip from it. You could say that the sponge had a water capacity. It could hold so much water before it couldn’t hold any more and the water started dripping out. Heat capacity is similar. Heat capacity is how much heat an object can absorb before it increases in temperature. This is also referred to as specific heat. Specific heat is how much heat energy a mass of a material must absorb before it increases 1°C.
Exercises Answer the questions below:
- What is specific heat?
- The specific amount of heat any object can hold
- The amount of energy required to raise the temperature of an object by 1 degree Celsius.
- The type of heat energy an object emits
- The speed of a compound’s molecules at room temperature
- Name two types of heat energy:
- What type (or types) of heat energy is at work in today’s experiment?
- True or False: Water is poor at absorbing heat energy.
- True
- False
Click here to go to next lesson on How much energy does a candy bar have?
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How much energy does a candy bar have? How much energy does a candy bar have? If you flip it over and read the nutritional information on the back, you can figure it out with a little help from the video below:
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Click here to go to next lesson on Heat Flow.
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Let’s learn how to calculate the heat flow based solely on temperature readings from a thermometer (this is going to be important later in this lab):
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Click here to go to next lesson on Heat and States of Matter.
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Heat can also change the state of matter. When an ice cube melts into a liquid puddle, it remains at the same temperature until the phase change is complete, and only then does the temperature begin to rise, even though heat was added throughout the entire process. The thermometer reading will stay on the same temperature reading until the ice is completely melted.
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Click here to go to next lesson on Sublimation.
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Carbon dioxide goes straight from a solid to a gas, which is called “sublimation”. It totally skips going through the liquid phase! How do you handle the transition from a solid block to a gas cloud? Here’s how:
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Did you know that eating a single peanut will power your brain for 30 minutes? The energy in a peanut also produces a large amount of energy when burned in a flame, which can be used to boil water and measure energy.
Click here to go to next lesson on Heat Energy of a Peanut.
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This experiment is for advanced students. Did you know that eating a single peanut will power your brain for 30 minutes? The energy in a peanut also produces a large amount of energy when burned in a flame, which can be used to boil water and measure energy.
Peanuts are part of the bean family, and actually grows underground (not from trees like almonds or walnuts). In addition to your lunchtime sandwich, peanuts are also used in woman’s cosmetics, certain plastics, paint dyes, and also when making nitroglycerin.
What makes up a peanut? Inside you’ll find a lot of fats (most of them unsaturated) and antioxidants (as much as found in berries). And more than half of all the peanuts Americans eat are produced in Alabama. We’re going to learn how to release the energy inside a peanut and how to measure it.
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Materials:
- raw peanuts
- chemistry stand with glass test tube and holder (watch video)
- flameproof surface (large ceramic tile or cookie sheet)
- paper clip
- alcohol burner or candle with adult help
- fire extinguisher
Download Student Worksheet & Exercises
What’s Going On? There’s chemical energy stored inside a peanut, which gets transformed into heat energy when you ignite it. This heat flows to raise the water temperature, which you can measure with a thermometer. You should find that your peanut contains 1500-2100 calories of energy! Now don’t panic… this isn’t the same as the number of calories you’re allowed to eat in a day. The average person aims to eat around 2,000 Calories (with a capital “C”). 1 Calorie = 1,000 calories. So each peanut contains 1.5-2.1 Calories of energy (the kind you eat in a day). Do you see the difference?
But wait… did all the energy from the peanut go straight to the water, or did it leak somewhere else, too? The heat actually warmed up the nearby air, too, but we weren’t able to measure that. If you were a food scientist, you’d use a nifty little device known as a bomb calorimeter to measure calorie content. It’s basically a well-insulated, well-sealed device that catches nearly all the energy and flows it to the water, so you get a much more accurate temperature reading. (Using a bomb calorimeter, you’d get 6.1-6.8 Calories of energy from one peanut!)
How do you calculate the calories from a peanut?
Let’s take an example measurement. Suppose you measured a temperature increase from 20 °C to 100 °C for 10 grams of water, and boiled off 2 grams. We need to break this problem down into two parts – the first part deals with the temperature increase, and the second deals with the water escaping as vapor.
The first basic heat equation is this:
Q = m c T
Q is the heat flow (in calories)
m is the mass of the water (in grams)
c is the specific heat of water (which is 1 degree per calorie per gram)
and T is the temperature change (in degrees)
So our equation becomes: Q = 10 * 1 * 80 = 800 calories.
If you measured that we boiled off 2 grams of water, your equation would look like this for heat energy:
Q = L m
L is the latent heat of vaporization of water (L= 540 calories per gram)
m is the mass of the water (in grams)
So our equation becomes: Q = 540 * 2 = 1080 calories.
The total energy needed is the sum of these two:
Q = 800 calories + 1080 calories = 1880 calories.
A peanut is not a nut, but actually a seed. In addition to containing protein, a peanut is rich in fats and carbohydrates. Fats and carbohydrates are the major sources of energy for plants and animals.
The energy contained in the peanut actually came from the sun. Green plants absorb solar energy and use it in photosynthesis. During photosynthesis, carbon dioxide and water are combined to make glucose. Glucose is a simple sugar that is a type of carbohydrate. Oxygen gas is also made during photosynthesis.
The glucose made during photosynthesis is used by plants to make other important chemical substances needed for living and growing. Some of the chemical substances made from glucose include fats, carbohydrates (such as various sugars, starch, and cellulose), and proteins.
Photosynthesis is the way in which green plants make their food, and ultimately, all the food available on earth. All animals and nongreen plants (such as fungi and bacteria) depend on the stored energy of green plants to live. Photosynthesis is the most important way animals obtain energy from the sun.
Oil squeezed from nuts and seeds is a potential source of fuel. In some parts of the world, oil squeezed from seeds-particularly sunflower seeds-is burned as a motor fuel in some farm equipment. In the United States, some people have modified diesel cars and trucks to run on vegetable oils.
Fuels from vegetable oils are particularly attractive because, unlike fossil fuels, these fuels are renewable. They come from plants that can be grown in a reasonable amount of time.
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Materials
- Shelled peanut
- Small pair of pliers
- Match or lighter
- Sink
Download Student Worksheet & Exercises
Procedure
ASK AN ADULT TO HELP YOU WITH THIS EXPERIMENT. DO NOT DO THIS EXPERIMENT BY YOURSELF. The fuel from the peanut can flare up and burn for a longer time than expected.
Close the drain in the kitchen sink. Fill the sink with water until the bottom of the sink is just covered.
Using a small pair of pliers, hold the peanut over the sink containing water. Ask an adult to hold the flame of a lit match or lighter directly under the peanut. When the peanut starts to burn, the lit match or lighter can be removed.
Allow the peanut to burn for one minute. MAKE SURE AN ADULT REMAINS PRESENT AND MAKE SURE TO HOLD THE PEANUT OVER THE SINK. To extinguish the burning peanut, drop it into the water. After you have extinguished the peanut, allow it to cool and then examine it carefully.
Observations
How long does it take for the peanut to start to burn? Does the peanut burn with a clean flame or a sooty flame? What color is the flame? What color does the peanut turn when it burns? Did the size of the peanut change after it has burned for several minutes?
Discussion
You should find that the peanut ignites and burns after a lit match or lighter is held under it for a few seconds. Although you only let the peanut burn for one minute as a safety measure, the peanut would burn for many minutes.
In this experiment, when the peanut burns, the stored energy in the fats and carbohydrates of the peanut is released as heat and light. When you eat peanuts, the stored energy in the fats and carbohydrates of the peanut is used to fuel your body.
Other Things to Try
Hold one end of a piece of uncooked spaghetti in a pair of pliers. Ask an adult to hold the flame of a lit match or lighter under the other end of the spaghetti. When the spaghetti starts to burn, place it in an aluminum pie pan that is in the sink. Make sure to extinguish the burning spaghetti with water when you are finished with the experiment. How does the burning of the spaghetti compare with the burning of the peanut?
Exercises
- What is the process called where plants get food from the sun?
- Osteoporosis
- Photosynthesis
- Chlorophyll
- Metamorphosis
- Where does all life on the planet get its food?
- List two ways that we could use the energy in a peanut:
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Click here to go to next lesson on Thermostat.
If you can remember thermostats before they went ‘digital’, then you may know about bi-metallic strips – a piece of material made from of two strips of different metals which expand at different rates as they are heated (usually steel and copper). The result is that the flat strip bends one way if heated, and in the opposite direction if cooled.
Normally, it takes serious skill and a red-hot torch to stick two different metals together, but here’s a homemade version of this concept that your kids can make using your freezer. Here what you do:
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Materials:
- foil wrapper from a stick of gum or candy bar
- index card
- scissors
- tape
Since gum wrappers are paper on one side and foil on the other, you can use one to make your own bi-metallic strip. Flatten out the wrapper into a sheet and find a way fasten the wrapper so it sits upright on an index card (we used the bubble gum itself as the adhesive). Stick it in the freezer overnight and check it in the morning! Where can you place it to flex the other direction?
How does that work? A bimetallic strip is a stack of two metals stuck together. The metal with the higher expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled. The bi-metallic strip was invented by the eighteenth century clockmaker John Harrison to compensate for temperature-induced changes his clock springs.
Click here to go to next lesson on Triple Point.
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The triple point is where a molecule can be in all three states of matter at the exact same time, all in equilibrium. Imagine having a glass of liquid water happily together with both ice cubes and steam bubbles inside, forever! The ice would never melt, the liquid water would remain the same temperature, and the steam would bubble up. In order to do this, you have to get the pressure and temperature just right, and it’s different for every molecule.
The triple point of mercury happens at -38oF and 0.000000029 psi. For carbon dioxide, it’s 75psi and -70oF. So this isn’t something you can do with a modified bike pump and a refrigerator.
However, the triple point of water is 32oF and 0.089psi. The only place we’ve found this happening naturally (without any lab equipment) is on the surface of Mars.
Because of these numbers, we can get water to boil here on Earth while it stays at room temperature by changing the pressure using everyday materials. (If you have a vacuum pump, you can have the water boil at the freezing point of 32oF.)
Here’s what you need to do:
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Materials:
- plastic syringe (no needle)
- room temperature water
Bonus Idea: Do this experiment first with water, then with carbonated water.
Why does that work? How did you get the pressure to decrease? Easy – when you pulled on the plunger and increased the volume inside the syringe. Since your finger covered the hole, no additional air was allowed in when you did this (which is why it was probably a little tough to do), so the number of molecules inside the syringe stayed the same, but the space they had to wiggle around got a lot bigger, meaning that the pressure decreased.
The air inside the syringe isn’t just plain old air… it has water vapor inside, too. And that’s not all – the water from your sink isn’t just plain old water, it has air bubbles mixed in with it. When you brought down the pressure (by pulling the plunger), you are forcing the air bubbles to come out of the water, which makes it boil. When you shove the plunger back in and increase the pressure, you’ll find that the air bubbles mix back into the water and disappears.
Did you try the soda water yet? Soda has carbon dioxide already mixed in for you, which is under pressure. You can release this pressure by opening the bottle (you’ll hear a PSSST!), which is the carbon dioxide bubbles coming out of the soda. Go ahead and try that now before reading further…
When you place the soda water into the syringe and decrease the pressure, the carbon dioxide comes out quickly Try tapping the syringe to make all the tiny bubbles combine into one larger bubble. When you increase the pressure (push the plunger back in), some of the bubbles will redissolve back into the soda.
If you’ve ever had a glass of hot water suddenly erupt in an explosion of bubbles, you’ve experienced superheated water (water that’s above it’s normal boiling point) that hasn’t been able to form bubbles yet. By adding a tea bag or simply just jiggling it around is usually enough to cause the bubbles to start, which often splatters HOT HOT water everywhere. (This isn’t something you want to try without adult help.)
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Click here to go to next lesson on Conduction.
In our example of the ice and the lemonade, it would work like this. The lemonade has a higher temperature than the ice. (The molecules are moving faster than the ice molecules.)
The faster moving molecules of the lemonade would transfer heat to the ice causing the ice molecules to move faster (increase temperature) and eventually change from solid to liquid.
In turn, since the faster moving molecules of the lemonade moves energy (transfers heat) to the ice, they slow down. This causes the temperature of your drink to decrease and that is what makes your lemonade nice and cold. Heat can be transferred in three different ways: conduction, convection and radiation.
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Let’s start with conduction. Heat is transferred through conduction the same way pool balls are scattered around a table in the opening break. On a pool table, one ball crashes into another ball which crashes into another ball speeding the balls up and moving them around the table.
Heat transferred from one object to another through conduction does the same thing. The molecules near the heat source (candle, stove, etc.) begin moving faster (their temperature increases).
Click here to go to next lesson on Convection.
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When something feels hot to you, the molecules in that something are moving very fast. When something feels cool to you, the molecules in that object aren’t moving quite so fast. Believe it or not, your body perceives how fast molecules are moving by how hot or cold something feels. Your body has a variety of antennae to detect energy. Your eyes perceive certain frequencies of electromagnetic waves as light. Your ears perceive certain frequencies of longitudinal waves as sound. Your skin, mouth and tongue can perceive thermal energy as hot or cold. What a magnificent energy sensing instrument you are!
Let’s find out how to watch the hot and cold currents in water. Here’s what you need to do:
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Materials:
- two bottles of water
- food coloring
- bathtub or sink
- index card or business card
You need:
Two empty bowls (or water bottles)
Food coloring
Hot water (Does not need to be boiling.)
Cold water
1. Put about the same amount of water into two bowls. One bowl should be filled with hot water from the tap. If you’re careful, you can put it in the microwave to heat it up but please don’t hurt yourself. The other bowl should have cold water in it. If you’re using water bottles, pour the hot and cold water into each bottle.
2. Let both bowls sit for a little bit (a minute or so) so that the water can come to rest.
3. Put food coloring in both bowls (or bottles) and watch carefully.
The food coloring should have spread around faster in the hot water bowl than in the cold water bowl. Can you see why? Remember that both bowls are filled with millions and millions of molecules. The food coloring is also bunches of molecules. Imagine that the molecules from the water and the molecules from the food coloring are crashing into one another like the beans on the plate. If one bowl has a higher temperature than the other, does one bowl have faster moving molecules? Yes, the higher temperature means a higher thermal energy. So the bowl with the warmer water has faster moving molecules which crash more and harder with the food coloring molecules, spreading them faster around the bowl.
If you’re using a bottle, you can do an extra step: For bottles, place a business card over the cold bottle and invert the cold bottle over the hot. Remove card.
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Click here to go to next lesson on Convection Currents.
Every time I’m served a hot bowl of soup or a cup of coffee with cream I love to sit and watch the convection currents. You may look a little silly staring at your soup but give it a try sometime!
Convection is a little more difficult to understand than conduction. Heat is transferred by convection by moving currents of a gas or a liquid. Hot air rises and cold air sinks. It turns out, that hot liquid rises and cold liquid sinks as well.
Room heaters generally work by convection. The heater heats up the air next to it which makes the air rise. As the air rises it pulls more air in to take its place which then heats up that air and makes it rise as well. As the air get close to the ceiling it may cool. The cooler air sinks to the ground and gets pulled back near the heat source. There it heats up again and rises back up.
This movement of heating and cooling air is convection and it can eventually heat an entire room or a pot of soup. This experiment should allow you to see convection currents.
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You need:
- A pot
- A stove with adult help
- Pepper
- Ice cubes
- Food Coloring (optional)
1. Fill the pot about half way with water.
2. Put about a teaspoon of pepper into the water.
3. Put the pot on the stove and turn on the stove (be careful please).
4. Watch as the water increases in temperature. You should see the pepper moving. The pepper is moving due to the convection currents. If you look carefully you many notice pepper rising and falling.
5. Put an ice cube into the water and see what happens. You should see the pepper at the top of the water move towards the ice cube and then sink to the bottom of the pot as it is carried by the convection currents.
6. Just for fun, put another ice cube into the water, but this time drop a bit of food coloring on the ice cube. You should see the food coloring sink quickly to the bottom and spread out as it is carried by the convection currents.
Did you see the convection currents? Hot water rising in some areas of the pot and cold water sinking in other areas of the pot carried the pepper and food coloring throughout the pot. This rising and sinking transferred heat through all the water causing the water in the pot to increase in temperature.
Heat was transferred from the flame of the stove to the water by convection. More accurately, heat was transferred from the flame of the stove to the metal of the pot by conduction and then from the metal of the pot throughout the water through convection.
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Click here to go to next lesson on Radiation.
Heat is transferred by radiation through electromagnetic waves. Remember, when we talked about waves and energy? Well, heat can be transferred by electromagnetic waves. Energy is vibrating particles that can move by waves over distances right? Well, if those vibrating particles hit something and cause those particles to vibrate (causing them to move faster/increasing their temperature) then heat is being transferred by waves. The type of electromagnetic waves that transfer heat are infra-red waves. The Sun transfers heat to the Earth through radiation.
If you hold your hand near (not touching) an incandescent light bulb until you can feel heat on your hand, you’ll be able to understand how light can travel like a wave. This type of heat transfer is called radiation.
Now don’t panic. This is not a bad kind of radiation like you get from x-rays. It’s infra-red radiation. Heat was transferred from the light bulb to your hand. The energy from the light bulb resonated the molecules in your hand. (Remember resonance?) Since the molecules in your hand are now moving faster, they have increased in temperature. Heat has been transferred! In fact, an incandescent light bulb gives off more energy in heat then it does in light. They are not very energy efficient.
Now, if it’s a hot sunny day outside, are you better off wearing a black or white shirt if you want to stay cool? This experiment will help you figure this out:
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You need:
- 2 ice cubes, about the same size.
- A white piece of paper
- A black piece of paper
- A sunny day
Download Student Worksheet & Exercises
1. Put the two pieces of paper on a sunny part of the sidewalk.
2. Put the ice cubes in the middle of the pieces of paper.
3. Wait.
What you should eventually see, is that the ice cube on the black sheet of paper melts faster then the ice cube on the white sheet. Dark colors absorb more infra-red radiation then light colors. Heat is transferred by radiation easier to something dark colored then it is to something light colored and so the black paper increased in temperature more then the white paper.
So, to answer the shirt question, a white shirt reflects more infra-red radiation so you’ll stay cooler. White walls, white cars, white seats, white shorts, white houses, etc. all act like mirrors for infra-red (IR) radiation. Which is why you can aim your TV remote at a white wall and still turn on the TV. Simply pretend the wall is a mirror (so you can get the angle right) and bounce the beam off the wall before it gets to your TV. It looks like magic!
Click over to this experiment to learn how to make Liquid Crystals.
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Click here to go to next lesson on Calorimeter.
You can’t get through a science or engineering degree without having performed a calorimetry lab. A calorimetry experiment is made of of inexpensive equipment (it only uses a coffee cup and a thermometer) and the calculations needed to do the experiment are pretty easy, so you can already tell that teachers are going to like them. They are useful in figuring out the specific heat capacity and the heat of fucion or dissolution of an unknown substance (usually a lump of metal). Here’s how to make a coffee cup calorimeter and do the calculations:
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Chemists use a bomb calorimeter to measure the heat flow for reactions that involve gases (since the gases would escape out of the coffee cup) and reactions at high temperatures (which would melt the cup). It works the same way as the coffee cup version, only the reaction is sealed and placed in water, which is then placed in an insulated container. It’s a more elaborate setup as well as analysis because now you take into account the heat flow into the parts of the calorimeter. Heat also does work as it transfers energy.
Click here to go to next lesson on Heat Engines.
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The Drinking Bird is a classic science toy that dips its head up and down into a glass of water. It’s filled with a liquid called methylene chloride, and the head is covered with red felt that gets wet when it drinks. But how does it work? Is it perpetual motion?
Let’s take a look at what’s going on with the bird, why it works, and how we’re going to modify it so it can run on its own without using any water at all!
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The bird needs a temperature difference between the head and tail. Since water needs heat in order to evaporate, the head cools as the water evaporates. This temperature decrease lowers the pressure inside the head, pushing liquid up the inner tube. With more liquid (weight in the head), the bird tips over. The bird wets its own head to start this cycle again.
The trick to making this work is that when the bird is tipped over, the vapor from the bottom moves up the tube to equalize the pressure in both sides, or he’d stay put with his head in the cup. Sadly, this isn’t perpetual motion because as soon as you take away the water, the cycle stops. It also stops if you enclose the bird in a jar so water can longer evaporate after awhile. Do you think this bird can work in a rainstorm? In Antarctica?
What’s so special about the liquid? Methylene chloride is made of carbon, hydrogen, and chlorine atoms. It’s barely liquid at room temperature, having a boiling point of 103.5° F, so it evaporates quite easily. It does have a high vapor pressure (6.7 psi), meaning that the molecules on the liquid surface leave (evaporate) and raise the pressure until the amount of molecules evaporating is equal to the amount being shoved back in the liquid (condensed) by its own pressure. (For comparison, water’s vapor pressure is only 0.4 psi).
Note that the vapor pressure will change with temperature changes. The vapor pressure goes up when the temperature goes up. Since the wet head is cooler than the tail, the vapor pressure at the top is less than at the bottom, which pushes the liquid up the tube.
It really does matter whether the bird is operating in Arizona or the Amazon. The bird will dip more times per minute in a desert than a rain forest!
Let’s find out how to modify the bird so it’s entirely solar-powered… meaning that you don’t have to remember to keep the cup filled with water. Here’s what you need:
- drinking bird
- silver or white spray paint
- black spray paint
- razor
- mug of hot water
- sunlight or incandescent light
Download Student Worksheet & Exercises
In this modification, you completely eliminated the water and converted the bird to solar, using the heat of the sun to power the bird. Now your bird bobs as long as you have sunlight!
How does that work? Since the bottom of the bird is now black, and black absorbs more energy and heats up the tail of the bird. Since the tail section is warmer, the pressure goes up and the liquid gets pushed up the tube. By covering the head with white (or silver) paint, you are reflecting most of the energy so it remains cool. Remember that white surfaces act like mirrors to IR light (which is what heat energy is).
Questions to Ask: Does it work better with hot or cold water? Does it work in an enclosed space, such as an inverted aquarium? On a rainy day or dry? In the fridge or heating pad?
Exercises Answer the questions below:
- Where does most of the energy on earth come from?
- Underground
- The sun
- The oceans
- What is one way that we use energy from the sun?
- What is the process by which the liquid is being heated inside the bird?
- Precipitation
- Pressure
- Evaporation
- Transpiration
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Click here to go to next lesson on Hero Engine.
There are lots of different kids of heat engines, from stirling engines to big jet turbines to the engine in your car. They all use clever ways to convert a temperature difference into motion.
Remember that the molecules in steam move around a lot faster than in an ice cube. So when we stick hot steam in a container, we can blow off the lid (used with pistons in a steam engine). or we can put a fan blade in hot steam, and since the molecules move around a lot, they start bouncing off the blade and cause it to rotate (as in a turbine). Or we can seal up hot steam in a container and punch a tiny hole out one end (to get a rocket).
One of the first heat engines was dreamed up by Hero of Alexandria called the aeolipile. The steam is enclosed in a vessel and allowed to jet out two (or more) pipes. Although we’re not sure if his invention ever made it off the drawing board, we do know how to make one for pure educational (and entertainment) purposes. Are you ready to have fun?
THIS EXPERIMENT USES FIRE AND STEAM…GET ADULT HELP BEFORE YOU OPERATE THE ENGINE.
Here’s what you do:
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Materials:
- soda can
- fishing line
- razor
- stand
- drill or large nail and hammer
- adult help
- candle
IMPORTANT NOTE: As the water boils, your can will spin. The hot steam shoots out the sides, so take care not to get burned. As the can spins, it may wobble and shake, especially if it’s off-center. Be careful not to get shot with boiling water!!
What’s going on? When the water boils, the molecules inside are turning into hot steam and moving very quickly, bouncing off the can and out the pipes. Rockets and balloons use this same principle – the pressurized air shoots out the open end and the balloon (or rocket) moves in the opposite direct. Newton’s third law in motion! The two jets at an angle work together to spin the can.
- Do you think the size of the hole matters?
- Does it matter how many candles you use?
- What if you used four holes instead of two? Six? Twenty?
- Are beer cans better?
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Click here to go to next lesson on Stirling Engine
This project is for advanced students.This Stirling Engine project is a very advanced project that requires skill, patience, and troubleshooting persistence in order to work right. Find yourself a seasoned Do-It-Yourself type of adult (someone who loves to fix things or tinker in the garage) before you start working on this project, or you’ll go crazy with nit-picky things that will keep the engine from operating correctly. This makes an excellent project for a weekend.
Developed in 1810s, this engine was widely used because it was quiet and could use almost anything as a heat source. This kind of heat engine squishes and expands air to do mechanical work. There’s a heat source (the candle) that adds energy to your system, and the result is your shaft spins (CD).
This engine converts the expansion and compression of gases into something that moves (the piston) and rotates (the crankshaft). Your car engine uses internal combustion to generate the expansion and compression cycles, whereas this heat engine has an external heat source.
This experiment is great for chemistry students learning about Charles’s Law, which is also known as the Law of Volumes, which describes how gases tend to expand when they are heated and can be mathematically written like this:
where V = volume, and T = temperature. So as temperature increases, volume also increases. In the experiment you’re about to do, you will see how heating the air causes the diaphragm to expand which turns the crank.
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Here’s what you need:
- three soda cans
- old inner tube from a bike wheel
- super glue and instrant dry
- electrical wire (3- conductor solid wire)
- 3 old CDs
- one balloon
- penny
- nylon bushing (from hardware store)
- alcohol burner (you can build one out of soda cans or Sterno canned heat)
- fishing line (15lb. test or similar)
- pack of steel wool
- drill with 1/16″ bit
- pliers
- scissors
- razor
- wire cutters
- electrical tape
- push pin
- permanent marker
- Swiss army knife (with can opener option)
- template
The Stirling heat engine is very different from the engine in your car. When Robert Stirling invented the first Stirling engine in 1816, he thought it would be much more efficient than a gasoline or diesel engine. However, these heat engines are used only where quiet engines are required, such as in submarines or in generators for sailboats.
Download Student Worksheet & Exercises
Here’s how a Stirling engine is different from the internal-combustion engine inside your car. For example, the gases inside a Stirling engine never leave the engine because it’s an external combustion engine. This heat engine does not have exhaust valves as there are no explosions taking place, which is why Stirling engines are quieter. They use heat sources that are outside the engine, which opens up a wide range of possibilities from candles to solar energy to gasoline to the heat from your hand.
There are lots of different styles of Stirling engines. In this project, we’ll learn about the Stirling cycle and see how to build a simple heat engine out of soda cans. The main idea behind the Stirling engine is that a certain volume of gas remains inside the engine and gets heated and cooled, causing the crankshaft to turn. The gases never leave the container (remember – no exhaust valves!), so the gas is constantly changing temperature and pressure to do useful work. When the pressure increases, the temperature also increases. And when the temperature of the gases decreases, the pressure also goes down. (How pressure and temperature are linked together is called the “Ideal Gas Law”.)
Some Stirling engines have two pistons where one is heated by an external heat source like a candle and the other is cooled by external cooling like ice. Other displacer-type Stirling engines has one piston and a displacer. The displacer controls when the gas is heated and cooled.
In order to work, the heat engine needs a temperature difference between the top and bottom of the cylinder. Some Stirling engines are so sensitive that you can simply use the temperature difference between the air around you and the heat from your hand. Our Stirling engine uses temperature difference between the heat from a candle and ice water.
The balloon at the top of the soda can is actually the ‘power piston’ and is sealed to the can. It bulges up as the gas expands. The displacer is the steel wool in the engine which controls the temperature of the air and allows air to move between the heated and cooled sections of the engine.
When the displacer is near the top of the cylinder, most of the gas inside the engine is heated by the heat source and gas expands (the pressure builds inside the engine, forcing the balloon piston up). When the displacer is near the bottom of the cylinder, most of the gas inside the engine cools and contracts. (the pressure decreases and the balloon piston is allowed to contract).
Since the heat engine only makes power during the first part of the cycle, there’s only two ways to increase the power output: you can either increase the temperature of the gas (by using a hotter heat source), or by cooling the gases further by removing more heat (using something colder than ice).
Since the heat source is outside the cylinder, there’s a delay for the engine to respond to an increase or decrease in the heat or cooling source. If you use only water to cool your heat engine and suddenly pop an ice cube in the water, you’ll notice that it takes five to fifteen seconds to increase speed. The reason is because it takes time for the additional heat (or removal of heat by cooling) to make it through the cylinder walls and into the gas inside the engine. So Stirling engines can’t change the power output quickly. This would be a problem when getting on the freeway!
In recent years, scientists have looked to this engine again as a possibility, as gas and oil prices rise, and exhaust and pollutants are a concern for the environment. Since you can use nearly any heat source, it’s easy to pick one that has a low-fume output to power this engine. Scientists and engineers are working on a model that uses a Stirling engine in conjunction with an internal-combustion engine in a hybrid vehicle… maybe we’ll see these on the road someday!
Exercises
- What is the primary input of energy for the Stirling engine?
- As Pressure increases in a gas, what happens to temperature?
- It increases
- Nothing
- It decreases
- It increases, then decreases
- What is the primary output of the Stirling engine?
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Click here to go to next lesson on Ideal Gas Law.
The ideal gas law is important because you can predict how most gases with behave with a simple equation. Here’s how to do it:
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We’re nearly done! Take a peek at the list of concepts below and make sure you’ve got a good handle on them…
Scientific Concepts:
- The terms hot, cold, warm etc. describe what physicists call thermal energy.
- Thermal energy is how much the molecules are moving inside an object.
- The faster molecules move, the more thermal energy that object has.
- There are different scales for measuring temperature: Fahrenheit, Celsius, Rankine and Kelvin.
- Temperature is basically a speedometer for molecules. The faster they are wiggling and jiggling, the higher the temperature and the higher the thermal energy that object has.
- Solids have strong, stiff bonds between molecules that hold the molecules in place. Liquids have loose, stringy bonds between molecules that hold molecules together but allow them some flexibility. Gasses have no bonds between the molecules. Plasma is similar to gas but the molecules are very highly energized. Materials change from one state to another depending on the temperature and these bonds.
- Changing from a solid to a liquid is called melting. Changing from a liquid to a gas is called boiling, evaporating, or vaporizing. Changing from a gas to a liquid is called condensation. Changing from a liquid to a solid is called freezing.
- All materials have given points at which they change from state to state.
- Melting point is the temperature at which a material changes from solid to liquid. Boiling point is the temperature at which a material changes from liquid to gas. Condensation point is the temperature at which a material changes from gas to liquid. Freezing point is the temperature at which a material changes from liquid to gas.
- Heat is the movement of thermal energy from one object to another. Heat can only flow from an object of a higher temperature to an object of a lower temperature.
- Heat can be transferred from one object to another through conduction, convection and radiation.
- Conduction is the wiggle and bump method of heat transfer. Faster moving molecules bump into slower moving molecules speeding them up. Those molecules then bump into other molecules speeding them up and so on increasing the temperature of the object.
- Convection is heat being transferred by currents of moving gas or liquid caused by hot air/liquid rising and cold air/liquid falling.
- Radiation is the transfer of heat by electromagnetic radiation, specifically infra-red radiation.
- When an object absorbs heat it does not necessarily change temperature. As objects change state they do not change temperature. The heat that goes into something as it’s changing phases is used to change the “bonds” between molecules. Freezing points, melting points, boiling points and condensation points are the “speed limits” of the phases. Once the molecules reach that speed they must change state.
- Objects release heat as they freeze and condense. Objects absorb heat as they evaporate and melt.
- Heat capacity is how much heat an object can absorb before its temperature increases.
- Specific heat is how much heat energy a mass of a material must absorb before it increases 1°C.
- Each material has its own specific heat. The higher a material’s specific heat is, the more heat it must absorb before its temperature increases. Water has a very high heat capacity.
Yay! You’ve completed the lessons in Thermodynamics! Now it’s time to try your own problem set.
Click here to download the problem set (with solutions).
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Momentum can be defined as inertia in motion. Something must be moving to have momentum. Momentum is how hard it is to get something to stop or to change directions. A moving train has a whole lot of momentum. A moving ping pong ball does not. You can easily stop a ping pong ball, even at high speeds. It is difficult, however, to stop a train even at low speeds.
Mathematically, momentum is mass times velocity, or p = mv.
Momentum is a vector quantity, because it’s based on velocity, so you’ll expect to have a number and a direction in your answer for momentum questions. The heavier something is and/or the faster it’s moving the more momentum it has. The more momentum something has, the more force it takes to get it to change velocity and the more force it can apply if it hits something.
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Now let’s discuss impulse. Impulse is a measure of force and time. Remember, force is a push or a pull, right? Well, impulse is how much force is applied for how much time. Mathematically it’s impulse equals force x time or impulse = Ft.
Click here to go to next lesson on Defined by Physics.
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Think about baseball. When you hit a baseball, do you just smack it with the bat or do you follow through with the swing? You follow through right? Do you see how impulse relates to your baseball swing? If you follow through with your swing, the bat stays in contact with the ball for a longer period of time. This causes the ball to go farther. Follow through is important in golf, bowling, tennis and many sports for the same reason. The longer the force is imparted, the farther and faster your ball will go.
Ok, let’s add impulse and momentum together and see what we get. Impulse changes momentum. If an object puts an impulse on another object, the momentum of both objects will change. If you continue to push on your stalled car, you will change the momentum of the car right? If you are riding your bike while not paying any attention and crash into the back of a parked car, you will put an impulse on the car and you and the car’s momentum will change. (As a kid, I did this pretty often. That’s what you get when you ride and wonder at the same time. Believe me when I tell you that my momentum changed a lot more than the car’s did!!)
In fact, there is a mathematical formula about this impulse and momentum thing. Impulse = change in momentum or Ft = change in mv. Force x time = mass x velocity. Does that sound familiar to anyone? It’s awfully similar to Newton’s second law (F=ma) isn’t it? In fact it’s the same thing: F t = m v
Now if we divide both sides by “t” we get F = mv / t.
Another way to say v is d/t (distance over time). So now we have F = m (d/t) / t. Those two “t’s” together are the same as t2 and d/t2 is “a” (acceleration). So what we have now is F = ma!
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Click here to go to next lesson on Windshield Problem.
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When you want to shrink the force of impact, you want to increase the time the impact takes. This is called the collision time. The longer the collision time the longer it takes your momentum to come to zero. Here’s the math.
If you are in a 1000 kg vehicle moving at 30 m/s your momentum is 1000 x 30 or 30,000 Ns ("Newton-seconds"). Now, lets say you hit a brick wall so your momentum goes from 30,000 to 0 in .5 seconds.
The equation we're going to use is the impulse-momentum change theorem: Ft = mv... so...
F x (0.5 seconds) = 30,000 Ns
so solve that equation for F= 60,000N!! That’s gonna leave a mark! Now lets say that instead of hitting a brick wall you hit a mound of hay and so the impact takes 3 seconds.
Now the formula looks like this: F x (3 seconds) = 30,000 Ns
and solve for force to get F= 10,000N.
Do you notice the difference? 60,000N versus 10,000N of force? All those safety features, seat belts, helmets, air bags, are designed to increase how long it takes your momentum to come to zero. Newton’s laws to the rescue! Let’s do a couple of experiments to help this information have more impact.
Click here to go to next lesson on Smacking Pennies.
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Here is a quick experiment… first, find a wall. Then hit it with your bare fist. (Take it easy, just hit it with enough force that you feel the impact.) Now put a pillow in front of the wall and hit it with about the same force as you hit it before. With the pillow in front of the wall, you can hit it a little harder if you like but again, don’t go nuts!
What did the pillow do? It slowed the time of impact. Remember our formula Ft = mv. When the momentum of your moving fist struck the wall directly, the momentum was cut to zero instantly and so you felt enough force to hurt a bit. When the pillow was in the way it took longer for your momentum to come to zero. So you could hit the pillow fairly hard without feeling much force. Basically a bike helmet is like a pillow for you head. It slows the time of impact, so when you fall off your bike, there is much less force on your head. Just be glad your mom doesn’t make you wear a pillow on your head!
So let’s go back to momentum for a minute. Momentum is inertia in motion. It is how much force it takes to get something to slow down or change direction. One more concept I’d like to give you this month, is conservation of momentum. This is basically momentum equals momentum or mathematically mv = mv. (Momentum is mass times velocity.) When objects collide, the momentum that both objects have after the collision, is equal to the amount of momentum the objects had before the collision. Let’s take a look at this with this experiment.
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The video above shows you another quick experiment you can do with momentum: Put one penny on the table. Put another penny on the table about 6 inches away from the first one. Now, slide one penny fairly fast towards the first penny. What you want to have happen, is that the moving penny strikes (or gives impulse to) the stationary penny head on. The moving penny should stop and the stationary penny will move. Now, try that with other coins. Make big ones hit small ones and vice versa. It’s also fun to put a line of 5 coins all touching one another. Then strike the end of the line with a moving penny.
This is conservation of momentum. If you were able to strike the penny head on, you should have seen that the penny that was moving, stopped, and the penny that was stationary moved with about the same speed of the original moving penny. Conservation of momentum is mv = mv. Once the moving penny struck the other, all the moving penny’s momentum transferred to the second penny. Since the pennies weighed the same, the v (velocity) of the first penny is transferred to the second penny and the second penny moves with the same velocity as the first penny. What happens if you use a quarter and a penny? Make the quarter strike the penny. That penny should really zip! Again mv = mv. The mass of the quarter is much greater then the mass of the penny. So for momentum to be conserved, after impact, the penny had to have a much greater velocity to compensate for its lower mass.
Mathematically it would look like this… after collision:
(Mass of Quarter) x (Velocity of Quarter) = (Mass of Penny) x (Velocity of Penny)
5g x 10m/s = 1g x v
50 = 1 x v
50/1 = v
50m/s = v
or 5g x 10m/s = 1g x 50 m/s
50 momentum = 50 momentum
After the collision, the penny is moving at 50 m/s, 5 times faster then the quarter was moving because the penny is 5 times lighter then the quarter.
Click here to go to next lesson on Wagons.
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Second Law of Motion: Momentum is conserved. Momentum can be defined as mass in motion. Something must be moving to have momentum. Momentum is how hard it is to get something to stop or to change directions. A moving train has a whole lot of momentum. A moving ping pong ball does not. You can easily stop a ping pong ball, even at high speeds. It is difficult, however, to stop a train even at low speeds.
Materials: garden hose connected to a water faucet
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Place your thumb partway over the end of a garden hose. The water shoots out faster because the same amount of “stuff” has to pass through the exit. When the exit area decreases, less mass can pass through at one time, so the velocity increases.
Mathematically, momentum is mass times velocity, or Momentum=mv.
One of the basic laws of the universe is the conservation of momentum. When objects smack into each other, the momentum that both objects have after the collision, is equal to the amount of momentum the objects had before the crash.
The next video shows you how once the two balls hit the ground, all the larger ball’s momentum transferred to the smaller ball (plus the smaller ball had its own momentum, too!) and thus the smaller ball goes zooming to the sky.
Download Student Worksheet & Exercises
Do you see how using a massive object as the lower ball works to your advantage here? What if you shrink the smaller ball even more, to say bouncy-ball size? Momentum is mass times by velocity, and since you aren’t going to change the velocity much (unless you try this from the roof, which has its own issues), it’s the mass that you can really play around with to get the biggest change in your results. So for momentum to be conserved, after impact, the top ball had to have a much greater velocity to compensate for the lower ball ’s velocity going to zero.
Find out more about this key principle in Unit 1 and Unit 2.
Advanced students: Download your Momentum lab here.
Exercises
- What concept does Newton’s Second Law of Motion deal with?
- What is momentum?
Click here to go to next lesson on Rebounding.
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A rebound is a special kind of collision where objects bounce off each other instead of sticking to each other. There’s a change in the direction and a speed change.
Imagine a tennis ball striking a brick wall. The ball initially has a sped of 10 m/s, and after it hits the wall, it bounces back in the opposite direction at half the speed. What is the velocity change? It’s 10+5 m.s or 15 m/s.
Would the acceleration be greater or less than a ball that rebounds with a speed of 8 m/s? (Greater, since acceleration depends on velocity change, and the change in velocity for the second throw is 12 m.s). Which has the greatest momentum change? (The first case, since momentum change depends on velocity change.)
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Let’s take a look at how you calculate the change of momentum. You’ll do a rebounding experiment soon, so let’s get familiar with the equations you’ll need when you get to that experiment.
Click here to go to next lesson on Elastic Collisions.
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Sometimes an object will have the same (or nearly the same) speed as it had before impact, and these are called elastic collisions. These kinds of collisions also have the same kinetic energy and same momentum before and after the collision.
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Click here to go to next lesson on Energy Transfer.
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Instant ball launcher!
You'll find the top ball rockets off skyward while the lower ball hit the floor flat (without bouncing much, if at all). Now why is that? It's easier to explain than you think...
Remember momentum? Momentum can be defined as inertia in motion. Something must be moving to have momentum. Momentum is how hard it is to get something to stop or to change directions. A moving train has a whole lot of momentum. A moving ping pong ball does not. You can easily stop a ping pong ball, even at high speeds. It is difficult, however, to stop a train even at low speeds.
Mathematically, momentum is mass times velocity, or Momentum=mv.
One of the basic laws of the universe is the conservation of momentum. When objects smack into each other, the momentum that both objects have after the collision, is equal to the amount of momentum the objects had before the crash. Once the two balls hit the ground, all the larger ball's momentum transferred to the smaller ball (plus the smaller ball had its own momentum, too!) and thus the smaller ball goes zooming to the sky.
Materials:
- two balls, one significantly larger than the other
Download Student Worksheet & Exercises
Do you see how using a massive object as the lower ball works to your advantage here? What if you shrink the smaller ball even more, to say bouncy-ball size? Momentum is mass times by velocity, and since you aren't going to change the velocity much (unless you try this from the roof, which has its own issues), it's the mass that you can really play around with to get the biggest change in your results. So for momentum to be conserved, after impact, the top ball had to have a much greater velocity to compensate for the lower ball 's velocity going to zero.
You can also try a small bouncy ball (about the size of a quarter) and a larger bouncy ball (tennis-ball size) and rest the small one on top of the large one. Hold upright as high as you can, then release. If the balls stay put (the small one stays on top of the larger) at impact, the energy transfer will create a SUPER high bounce for the small ball. (Note how high the larger ball bounces when dropped.)
What happens if you try THREE?
Read more about impulse here.
Exercises
- What is the mathematical formula for momentum?
- Explain momentum in words.
- What happens to the momentum of the bottom ball in this experiment?
Click here to go to next lesson on Newton's Laws of Motion .
[/am4show]The Third Law of Motion shows up in collisions between objects. When two objects hit each other, they experience forces of the same magnitude but in opposite directions at impact. Those forces cause one object to speed up and the other to slow down. Even though the forces between the two objects are equal in magnitude, their accelerations are not.
Newton’s Second Law of Motion states that acceleration depends on force and mass, which means if you smack a ping pong ball with a bowling ball, one is going to have a higher acceleration than the other after the collision.
Golfers and baseball players use this principle to drive the ball far from their collision point by swinging the club or bat at high speeds, and even though the ball and bat experience the same force (in magnitude) at impact, the acceleration of the ball is much higher than the bat because the ball has a much lower mass. If you’re playing pool, then you can expect the billiard balls to experience the same accelerations after impact since the balls are all the same mass.
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Click here to go to next lesson on The Conservation of Momentum.
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The Conservation of Momentum tells us that the total momentum of a system (a set of objects) is a constant value that doesn’t change. The total momentum of two objects before the collision is equal to the total mo momentum of the two after the collision. The momentum lost by one object is gained by the other. You can think about momentum as money being exchanged between two people. If each person has $20, and one person gives the other $5, the money transfers from one person to the other. The money lost by the first person is gained by the other, but the total amount of money is the same before and after the transaction ($40).
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If someone stole $10 from one of them, then momentum is not conserved, which means that your system wasn’t truly isolated. For momentum to be conserved, you must be using an isolated system. Things like air resistance and friction steal momentum from a system. Since air resistance and friction are part of every system you’re going to analyze, you might be wondering how to use the conservation of momentum! One of the things in science we like to use is “assumptions”.
For example, for a lot of problems, we can assume air resistance and/or friction don’t come into play because compared with the total momentum of the system, they account for less than 2%. Whenever you’re going to neglect something, make sure you first compare it to the rest of the forces and motion of your system to find out if it’s going to matter much or not if you overlook it. Usually the contact forces during collisions are large compared to frictional or drag forces.
Click here to go to next lesson on Rail Accelerator.
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Physics isn’t all about equations, though. Here’s a real experiment you can do with a couple of steel ball bearings, a strong magnet, and a toilet paper tube:
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You can take this project a step further by adding a magnetic force to accelerate the balls to a faster speed when they collide with the magnet, and thus transfer even more kinetic energy at impact:
Click here to go to next lesson on Explosions.
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When the cannon exploded in the problem above (in the video), the momentum of the total system is conserved. Now remember, that system is two objects: the cannon and the cannon ball. Before it exploded, the total momentum of the system is zero (nothing is moving yet). But after it fires, the total momentum of the system must still be zero for conservation of momentum. If the ball moves with 100 units of momentum to the right, then the cannon moves with 100 units of momentum to the left, so they always sum to zero.
Newton actually expressed his second law of motion in terms of momentum by stating that the rate of change of the momentum of a particle is proportional to the net force acting on the particle and is in the direction of that force.
Click here to go to next lesson on Light Speed.
[/am4show]Particles that move close to the speed of light have a different equation for momentum in order for momentum to be conserved using Einstein’s relativistic equations. The speeds of large objects like baseballs, bullets, and satellites are so much less than the speed of light so we can use Newton’s equations for it. If you’re studying electrons and other subatomic particles, you must use equations from special relativity.
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Click here to go to next lesson on Center of Mass.
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Sometimes it’s easiest to solve the problem by shrinking all the objects down to tiny particles. But in order to do that, you have to account for how lumpy and heavy (or light) your object. A baseball bat doesn’t balance in the exact middle of the bat. You have to account for the fact that the grip is skinnier than the end you hit with. But how would you figure that out? Here’s how…
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Click here to go to next lesson on Non-constant Mass.
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Highlights for Momentum & Impulse
- Impulse is the amount of time a force is put on an object. How hard and how long something gets pushed or pulled.
- Ft = Impulse. Impulse affects the momentum of an object.
- Momentum is inertia in motion, how hard it is to get something to change directions or speed. Momentum = mv.
- Conservation of momentum; mv = mv. If something hits something else the momentum of the objects before the collision will equal the momentum of the objects after the collision.
Download your Momentum Problem Set here.
[/am4show]If a particle moves in only one dimension, like a train on as straight track, it’s easy to answer the question about where it is because there’s only one component to it: “13m North” or “-3.6 feet.” It’s a single number with units and a positive or negative sign… that’s it. Pretty simple, right?
Well, the truth is that most objects move in two or three dimensions, and so we need more information to tell us where that object is, so we use vectors. We’re going to focus on objects moving in a two dimensional plane.
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Click here to go to next lesson on Two Dimensional Motion.
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We’re going to study particles (or projectiles) that move in two dimensions. This can be a cannon ball after being fired, a baseball after being thrown, a golf ball after being hit, a soccer ball after being kicked, or any other situation you can think of where an object is under the influence of only gravity after the initial force applied to move the object. (Usually we ignore wind resistance when we do these types of problems.)
The FBD of projectiles is simply a downward pointing arrow to indicate the weight. If it looks strange to have a force not in the direction of the object’s travel path, just remember that a force isn’t needed to sustain motion… it’s actually the opposite! Objects stop moving because of the forces applied to it. The FBD are always a snapshot of the forces acting on the object in that moment. The object can be moving in one direction and the force acting in another.
A projectile is a particle that is only experiencing gravity, and in most cases, gravity is only acting in one direction. Gravity doesn’t influence the horizontal motion (if we accounted for air resistance, then there would be a force in this direction as well), only the vertical motion. That’s why the ball falls to the ground when you throw it.
This means that a bullet fired horizontally from a gun experiences a constant horizontal velocity and a downward vertical acceleration. A bullet fired from a gun pointed up at a 45 degree angle also experiences a constant horizontal velocity and a downward vertical acceleration. A bullet fired from a gun in outer space away from any gravitational influences would travel up at a 45 degree path away from the gun and experience constant horizontal and vertical velocity.
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The path a projectile makes is parabolic, meaning that it follows the shape of a parabola. The horizontal motion of the projectile is independent of the vertical motion. You’ll need to think about each component as separate and independent.
2-D Kinematic Equations
vx = vxo + axt
vy = vyo + ayt
x = xo + vxot + 1/2 axt2
y = yo + vyot + 1/2 ayt2
vx2 = vxo2 + 2 ax (x – xo)
vy2 = vyo2 + 2 ay (y – yo)
We can transform the above equations into a set of equations specifically for projectile motion by setting the acceleration in the x direction equal to zero for constant velocity (ax = 0) and setting the acceleration in the y direction equal to gravity (ay = -g) and rewrite to get:
vx = vxo
vy = vyo – gt
x = xo + vxot
y = yo + vyot – 1/2 gt2
vy2 = vyo2 – 2 g (y – yo)
Click here to go to next lesson on Using Trigonometry with Physics.
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Now let’s do a set of physics problems so you can really see how to solve these. The first one shows you how to not only calculate an angle buried in a trig function, but also that you don’t need fancy equations to solve a problem and that you really have to understand what the problem is asking for, so you don’t waste time calculating stuff you don’t need.
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Click here to go to next lesson on Soccer Ball Science.
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This problem will show you how a soccer ball can also be a projectile, and how by knowing a couple of simple things, you can find out everything you need about the problem, including how far and how high the ball traveled in addition to its time of flight.
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Click here to go to next lesson on P-Shooter Launcher.
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This is a simple, fun, and sneaky way of throwing tiny objects. It’s from one of our spy-kit projects. Just remember, keep it under-cover. Here’s what you need:
- a cheap mechanical pencil
- two rubber bands
- a razor with adult help
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Advanced students: Download your P-Shooter Lab here.
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Click here to go to next lesson on Pirate Problem.
Okay now, back to work! Here’s a neat problem involving a pirate ship and a cannon ball. I seriously doubt pirates would be able to calculate this kind of problem when being fired at by a fortress, but you might have a captain that had a good sense based on experience of how far and fast that cannon ball could travel.
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Click here to go to next lesson on Easy to Build Catapult.
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When you drop a ball, it falls 16 feet the first second you release it. If you throw the ball horizontally, it will also fall 16 feet in the first second, even though it is moving horizontally… it moves both away from you and down toward the ground. Think about a bullet shot horizontally. It travels a lot faster than you can throw (about 2,000 feet each second). But it will still fall 16 feet during that first second. Gravity pulls on all objects (like the ball and the bullet) the same way, no matter how fast they go.
What if you shoot the bullet faster and faster? Gravity will still pull it down 16 feet during the first second, but remember that the surface of the Earth is round. Can you imagine how fast we’d need to shoot the bullet so that when the bullet falls 16 feet in one second, the Earth curves away from the bullet at the same rate of 16 feet each second?
Answer: that bullet needs to travel nearly 5 miles per second. (This is also how satellites stay in orbit – going just fast enough to keep from falling inward and not too fast that they fly out of orbit.)
Catapults are a nifty way to fire things both vertically and horizontally, so you can get a better feel for how objects fly through the air. Notice when you launch how the balls always fall at the same rate – about 16 feet in the first second. What about the energy involved?
When you fire a ball through the air, it moves both vertically and horizontally (up and out). When you toss it upwards, you store the (moving) kinetic energy as potential energy, which transfers back to kinetic when it comes whizzing back down. If you throw it only outwards, the energy is completely lost due to friction.
The higher you pitch a ball upwards, the more energy you store in it. Instead of breaking our arms trying to toss balls into the air, let’s make a simple machine that will do it for us. This catapult uses elastic kinetic energy stored in the rubber band to launch the ball skyward.
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Here’s what you need:
- 9 tongue-depressor size popsicle sticks
- four rubber bands
- one plastic spoon
- ping pong ball or wadded up ball of aluminum foil (or something lightweight to toss, like a marshmallow)
- hot glue gun with glue sticks
Download Student Worksheet & Exercises
What’s going on? We’re utilizing the “springy-ness” in the popsicle stick to fling the ball around the room. By moving the fulcrum as far from the ball launch pad as possible (on the catapult), you get a greater distance to press down and release the projectile. (The fulcrum is the spot where a lever moves one way or the other – for example, the horizontal bar on which a seesaw “sees” and “saws”.)
Troubleshooting: These simple catapults are quick and easy versions of the real thing, using a fulcrum instead of a spring so kids don’t knock their teeth out. After making the first model, encourage kids to make their own “improvements” by handing them additional popsicle sticks, spoons, and glue sticks (for the hot glue guns).
If they get stuck, you can show them how to vary their models: glue a second (or third, fourth, or fifth) spoon onto the first spoon for multi-ammunition throws, increase the number of popsicle sticks in the fulcrum from 7 to 13 (or more?), and/or use additional sticks to lengthen the lever arm. Use ping pong balls as ammo and build a fort from sheets, pillows, and the backside of the couch.
Want to make a more advanced catapult?
This catapult requires a little more time, materials, and effort than the catapult design above, but it’s totally worth it. This device is what most folks think of when you say ‘catapult’. I’ve shown you how to make a small model – how large can you make yours?
This project lends itself well to taking data and graphing your results: you and your child can jot down the distance traveled along with time aloft with further calculations for high school students for velocity and acceleration. My university students would also calculate statistics, percent error, and more. My students also mapped out the material properties of the ‘cantilevered beam’ as well as model the popsicle stick as a spring (to determine the spring constant (k) for your calculations from Hooke’s Law). You can take this project as far as you want, depending on the interest and ability of kids.
Materials:
- plastic spoon
- 14 popsicle sticks
- 3 rubber bands
- wooden clothespin
- straw
- wood skewer or dowel
- scissors
- hot glue gun
Try different ball weights (ping pong, foil crumpled into a ball, whiffle balls, marshmallows, etc) and chart out the results: make a data table that shows what ball you tried and how far it went. You can also use a stopwatch to time how long your ball was in the air.
You can also graph your results: make a chart where you plot each data point on a graph that has distance on the vertical axis and time on the horizontal axis.
Advanced Teaching Tips: For high school and college-level physics classes, you can easily incorporate these launchers into your calculations for projectile motion. Offer students different ball weights (ping pong, foil crumpled into a ball, and whiffle balls work well) and chart out the results.
Exercises Answer the questions below:
- How is gravity related to kinetic energy?
- Gravity creates kinetic energy in all systems.
- Gravity explains how potential energy is created.
- Gravity pulls an object and helps its potential energy convert into kinetic energy.
- None of the above
- If you could use your catapult to launch your ball of foil into orbit, how high would it have to go?
- Above the atmosphere
- High enough to slingshot around the moon
- High enough so that when it falls, the earth curves away from it
- High enough so that it is suspended in empty space
- Where is potential energy the greatest on the catapult?
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Click here to go to next lesson on Two Body Problems.
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This is a more advanced catapult design that uses more sticks, but it still shoots a good distance :
When you drop a ball, it falls 16 feet the first second you release it. If you throw the ball horizontally, it will also fall 16 feet in the first second, even though it is moving horizontally… it moves both away from you and down toward the ground. Think about a bullet shot horizontally. It travels a lot faster than you can throw (about 2,000 feet each second). But it will still fall 16 feet during that first second. Gravity pulls on all objects (like the ball and the bullet) the same way, no matter how fast they go.
What if you shoot the bullet faster and faster? Gravity will still pull it down 16 feet during the first second, but remember that the surface of the Earth is round. Can you imagine how fast we’d need to shoot the bullet so that when the bullet falls 16 feet in one second, the Earth curves away from the bullet at the same rate of 16 feet each second?
Answer: that bullet needs to travel nearly 5 miles per second. (This is also how satellites stay in orbit – going just fast enough to keep from falling inward and not too fast that they fly out of orbit.)
Catapults are a nifty way to fire things both vertically and horizontally, so you can get a better feel for how objects fly through the air. Notice when you launch how the balls always fall at the same rate – about 16 feet in the first second. What about the energy involved?
When you fire a ball through the air, it moves both vertically and horizontally (up and out). When you toss it upwards, you store the (moving) kinetic energy as potential energy, which transfers back to kinetic when it comes whizzing back down. If you throw it only outwards, the energy is completely lost due to friction.
The higher you pitch a ball upwards, the more energy you store in it. Instead of breaking our arms trying to toss balls into the air, let's make a simple machine that will do it for us. This catapult uses elastic kinetic energy stored in the rubber band to launch the ball skyward.
[am4show have='p8;p9;p15;p42;p151;p75;p85;p88;p92;' guest_error='Guest error message' user_error='User error message' ] Here's what you need:
- 9 tongue-depressor size popsicle sticks
- four rubber bands
- one plastic spoon
- ping pong ball or wadded up ball of aluminum foil (or something lightweight to toss, like a marshmallow)
- hot glue gun with glue sticks
Download Student Worksheet & Exercises

Troubleshooting: These simple catapults are quick and easy versions of the real thing, using a fulcrum instead of a spring so kids don’t knock their teeth out. After making the first model, encourage kids to make their own “improvements” by handing them additional popsicle sticks, spoons, and glue sticks (for the hot glue guns).
If they get stuck, you can show them how to vary their models: glue a second (or third, fourth, or fifth) spoon onto the first spoon for multi-ammunition throws, increase the number of popsicle sticks in the fulcrum from 7 to 13 (or more?), and/or use additional sticks to lengthen the lever arm. Use ping pong balls as ammo and build a fort from sheets, pillows, and the backside of the couch.
Want to make a more advanced catapult?
This catapult requires a little more time, materials, and effort than the catapult design above, but it's totally worth it. This device is what most folks think of when you say 'catapult'. I've shown you how to make a small model - how large can you make yours?This project lends itself well to taking data and graphing your results: you and your child can jot down the distance traveled along with time aloft with further calculations for high school students for velocity and acceleration. My university students would also calculate statistics, percent error, and more. My students also mapped out the material properties of the 'cantilevered beam' as well as model the popsicle stick as a spring (to determine the spring constant (k) for your calculations from Hooke's Law). You can take this project as far as you want, depending on the interest and ability of kids.
Materials:
- plastic spoon
- 14 popsicle sticks
- 3 rubber bands
- wooden clothespin
- straw
- wood skewer or dowel
- scissors
- hot glue gun
Try different ball weights (ping pong, foil crumpled into a ball, whiffle balls, marshmallows, etc) and chart out the results: make a data table that shows what ball you tried and how far it went. You can also use a stopwatch to time how long your ball was in the air.
You can also graph your results: make a chart where you plot each data point on a graph that has distance on the vertical axis and time on the horizontal axis.
Advanced Teaching Tips: For high school and college-level physics classes, you can easily incorporate these launchers into your calculations for projectile motion. Offer students different ball weights (ping pong, foil crumpled into a ball, and whiffle balls work well) and chart out the results.
Exercises Answer the questions below:
- How is gravity related to kinetic energy?
- Gravity creates kinetic energy in all systems.
- Gravity explains how potential energy is created.
- Gravity pulls an object and helps its potential energy convert into kinetic energy.
- None of the above
- If you could use your catapult to launch your ball of foil into orbit, how high would it have to go?
- Above the atmosphere
- High enough to slingshot around the moon
- High enough so that when it falls, the earth curves away from it
- High enough so that it is suspended in empty space
- Where is potential energy the greatest on the catapult?
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Click here to go to next lesson on Advanced Catapult.

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Click here to go to next lesson on Calculus and What It's Useful For.
[/am4show]So now let’s look ahead and sneak a peek into your future. Are you nervous about taking Calculus? Or if you have, have you wondered what Calculus could possibly be useful for? Here’s a two part video that shows you what Calculus is (and will even have you doing it before the end of the second video!) and how it’s used all the time in physics. Sir Isaac Newton was so frustrated that he couldn’t solve his physics problems with the math that was already developed at that time (algebra) that he set them aside to invent a branch of mathematics that could support his work in science, and that’s where Calculus came from. Here’s how we use it today in physics…
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Click here to go to next lesson on Rabbit Problem.
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Now that you know what functions are, here’s how to solve the rabbit problem:
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Click here to go to next lesson on Trebuchet.
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This experiment is for Advanced Students. For ages, people have been hurling rocks, sticks, and other objects through the air. The trebuchet came around during the Middle Ages as a way to break through the massive defenses of castles and cities. It’s basically a gigantic sling that uses a lever arm to quickly speed up the rocks before letting go. A trebuchet is typically more accurate than a catapult, and won’t knock your kid’s teeth out while they try to load it.
Trebuchets are really levers in action. You’ll find a fulcrum carefully positioned so that a small motion near the weight transforms into a huge swinging motion near the sling. Some mis-named trebuchets are really ‘torsion engines’, and you can tell the difference because the torsion engine uses the energy stored in twisted rope or twine (or animal sinew) to launch objects, whereas true trebuchets use heavy counterweights.
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This is a serious wood-construction project. If you have access to scrap wood and basic tools (and glue!), you have everything you need to build this project. You will need to find heavy objects (like rocks or marbles) for the weights.
We want kids to discover that science isn’t in the special parts that come with a kit, but rather in the imagination and skill of the kid building it. We strive to avoid parts that are specially made just for a kit, molded plastic pieces, etc. and instead use parts that any kid could buy from the store. This means that kids can feel free to change things around, use their own ideas to add improvements and whatever else their imagination can come up with. So on this note, let’s get started.
WARNING: This project requires the use of various hand tools. These tools should only be used with adult supervision, and should not be used by children under 12 years of age.
Tools you’ll need:
- Hammer
- Electric drill with ¼” bit
- Hot glue gun & glue sticks
- Measuring tape or ruler
- Hand saw & clamp (or miter box)
- Scissors
- Screwdriver (flathead) or wood chisel
Materials:
- 7 pieces of ½” x ½” x 24″ pieces of wood stock
- 2 pieces of ¾” x 24″ wood
- 1 piece of 3″ x 24″ wood
- 18″ Wooden dowel
- Screw eye
- Nails
- String
- Clear tube
- Rubber mesh
Note: wood pieces may be slightly larger or smaller than specified. Just use your best judgment when sizing.
From ½” x ½” x 24″ pieces of wood stock cut:
- 3 pieces 5″ long
- 2 pieces 9″ long
- 3 pieces 3-1/2″ long
- 4 pieces 5-1/2″ long
From the dowel cut:
- 2 pieces 7″ long
- 1 piece 4″ long
From the 3″ x 24″ flat piece of wood cut:
- 2 pieces 3″ long (one of these has a 1″ square notch in it)
- 2 pieces 5″ long
- 1 piece 4-1/2″ long
AND…
- String should be cut into 2 pieces 14-16″ long
- The pouch is cut from the rubber mesh and is 5″ x 1-1/2″
Advanced Students: Download your Student Worksheet Lab here!
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Click here to go to next lesson on Helpful Hints.
Yay! You've completed this section! Now it's your turn to solve your own set of physics problems:
Click here to download your problem set for projectile motion.
Vectors are different from scalar numbers because they also include information about direction. Velocity, acceleration, force, and displacement are all vectors. Speed, time, and mass are all scalar quantities. Acceleration can be either a scalar or a vector, although in physics it’s usually considered a vector. For example, a car traveling at 45 mph is a speed, whereas a car traveling 45 mph NW is a vector. When you draw a vector, it’s an arrow that has a head and a tail, where the head points in the direction the force is pulling or the object is moving.
The coordinate system you use can be a compass (north, south, east and west) which is good for problems involving maps and geography, rectangular coordinates (x and y axes) which is good for most problems with objects traveling in two directions, or polar coordinates (radius and angle) which is good for objects that spin or rotate.
We have to get really good at vectors and modeling real world problems down on paper with them, because that’s how we’ll break things down to solve for our answers. If you’re already comfortable with vectors, feel free to skip ahead to the next lesson. If you find you need to brush up or practice a little more, this section is for you.
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The next four videos are a review of what we’ve covered so far with vectors. If you jumped here without going through the first two sections on 1-D Kinematics or Newton’s Laws, watch these four videos now to get an overview of vector components, resultants, trigonometry, resolution, and component addition. If you’ve already worked through these, then skip down to the section on relative velocity and start there.
Here’s a basic introduction to scalars and vectors:
Click here to go to next lesson on Resultants.
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A resultant is the vector sum of all of the vectors, usually force vectors. You can’t just add the numbers (magnitudes) together! You have to account for the direction that you’re pushing the box in. Here’s what you need to know about vector diagrams and how to add vectors together:
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Click here to go to next lesson on Components.
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A vector in two dimensions has components in both directions. Here’s how to add vectors together to get a single resultant vector using component addition as well as trigonometry (the law of cosines and the law of sines):
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Click here to go to next lesson on Pythagorean Theorem.
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Vectors can be added together using the Pythagorean theorem if they are at right angles with each other (which components always are). Here’s more practice is how to do both rectangular and polar coordinate system components of a vector:
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Click here to go to next lesson on Relative Motion.
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deals with problems where one object moves with respect to another. For example, an airplane might be traveling at 300 knots according to its airspeed indicator, but since it has a 20 knot headwind, the speed you see the airplane traveling at is actually 280 knots. You’ve seen this in action if you’ve ever noticed a bird flapping its wings but not moving forward on a really windy day. In that case, the velocity of the wind is equal and opposite to the bird’s velocity, so it looks like the bird’s not moving.
But what if the airplane encounters a crosswind? Something that’s not straight-on light a head or tail wind? Here’s how you break it down with vectors:
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Click here to go to next lesson on Boat Problem.
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These types of problems aren’t limited to airplanes, though. Have you ever gone in a boat and drifted off course? Here’s what was happening from a physics point of view:
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Click here to go to next lesson on Crossing a River.
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These types of boat problems usually ask for the following information to be calculated: what is the resultant velocity of the boat, how much time does it take to cross the river, and what distance does the boat drift off course due to the wind? Let’s practice this type of problem again so you really can get the hang of it.
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Click here to go to next lesson on Hot Air Balloon.
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Wow! If you followed all that, you have a good working understanding of how to use math (like vectors and equations) to solve real world problems! Don't forget - the most important thing you do is READING and UNDERSTANDING the problem. Don't get hooked by shiny equations and spiffy calculations, when sometimes the answer is as simple as dividing one number by another. I can't tell you how many students make this physics stuff way harder than it has to be because they're sure they have to use fancy stuff to get the right answer. They waste time they could have spend doing fun stuff (like science experiments!) struggling over getting equations to fit together without understanding what those equations represent in the first place.
Click here to go to next lesson on Projectile Motion.
[/am4show]The best way to learn how to solve physics problems is to solve physics problems. You can’t just read about it and think about it in your head… you actually have to do it, just like riding a bike. You can read all about bicycles, how they work and what the individual parts do, but until you sit in the seat and try to ride the thing, it’s really hard to understand. I am going to do a series of different sample physics problems in the videos below and explain everything in detail so you can really see how to apply Newton’s Laws of Motion to problems in the real world.
After you’re done watching the samples, download your practice problem set (at the end of the lessons) and try it yourself!
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Here are a couple of hints on how to solve problems involving Newton’s Laws of Motion:
Click here to go to next lesson on Sled Problem.
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Sleds are great to practice physics problems with, because there’s no friction associated with the problem (it’s sitting on ice, not on the ground). This is a good one to start with to get used to how we use the kinematic equations along with Newton’s laws and FBD’s to solve real problems.
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Click here to go to next lesson on Reference Frames and the Truck Problem .
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This is a really common thing to see happen in the real world, and one that people have a hard time seeing from the point of view of an outside observer just sitting on the side of the road. If you’ve ever been in a truck where this happened to you, now you know why.
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Click here to go to next lesson on Rubber Tire Problem.
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Here’s a good example of how non-moving objects can be analyzed for missing components by setting the acceleration term in Newton’s second law to zero. (Although I’ve never tried this one, I can only imagine that in the real world, the tire would actually be moving.)
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Click here to go to next lesson on Using Newton’s Laws Together.
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This is a good example of Newton’s second and third laws in action and how to use both laws to help you solve a problem…
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Click here to go to next lesson on Chandelier Problem.
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Imagine this one is a chandelier hanging from the ceiling, and you want to find out if your cables are strong enough…
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Click here to go to next lesson on Breaking Loose.
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This is a great example of how to calculate forces for a static (no motion) system, and then what happens if you break loose and allow motion to happen. Note how the coordinate system was oriented to make the math a lot easier.
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Click here to go to next lesson on Pulley Problem.
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Pulley problems are common in physics, and in this example you will learn how to draw FBD with different coordinate systems that work with each drawing individually.
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Click here to go to next lesson on How to Gain and Lose Weight.
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Yay! You've completed the section! Now it's time for you to try solving these on your own:
Download your Practice Problem set here!
[/am4show]Question: why does the balloon race all over the room? The answer is because of something called 'thrust vectoring', which means you can change the course of the balloon by angling the nozzle around. Think of the kick you'd feel if you tried to angle around a fire hose operating at full blast. That kick is what propels balloons and fighter aircraft into their aerobatic tricks.
We're going to perform several experiments here, each time watching what's happening so you get the feel for the Third Law. You will need to find:
- balloons
- string
- wood skewer
- two straws
- four caps (like the tops of milk jugs, film canisters, or anything else round and plastic about the size of a quarter)
- wooden clothespin
- a piece of stiff cardboard (or four popsicle sticks)
- hot glue gun
[am4show have='p8;p9;p12;p39;p109;p72;p92;p95;' guest_error='Guest error message' user_error='User error message' ] 1. Blow up the balloon (don’t tie it)
2. Let it go.
3. Wheeeee!
4. Tie one end of the string to a chair.
5. Blow up the balloon (don’t tie it).
6. Tape a straw to it so that one end of the straw is at the front of the balloon and the other is at the nozzle of the balloon.
7. Thread the string through the straw and pull the string tight across your room.
8. Let go. With a little bit of work (unless you got it the first time) you should be able to get the balloon to shoot about ten feet along the string.
This is a great demonstration of Newton’s Third Law - the air inside the balloon shoots one direction, and the balloon rockets in the opposite direction. It’s also a good opportunity to bring up some science history. Many folks used to believe that it would be impossible for something to go to the moon because once something got into space there would be no air for the rocket engine to push against and so the rocket could not “push” itself forward.
In other words, those folks would have said that a balloon shoots along the string because the air coming out of the balloon pushes against the air in the room. The balloon gets pushed forward. You now know that that’s silly! What makes the balloon move forward is the mere action of the air moving backward. Every action has an equal and opposite reaction.
Multi-Stage Balloon Rocket
You can create a multi-stage balloon rocket by adding a second balloon to the first just like you see here in the video:Tie a length of string through the room, having at least twenty feet of clear length. Thread two straws onto the string before securing the end. Punch the bottoms out of two foam coffee cups and tape parallel to the threaded straws.
Blow up balloons while they are inside the cups, so they extend out either end. When blowing up the second balloon, sandwich the untied end of the first inflated balloon between the second inflated balloon surface and inside the cup. Hold the second balloon’s end with a clothespin and release!
Balloon Racecar
Now let's use this information to create a balloon-powered racecar. You'll need the rest of the items outlined above to build your racecar. NOTE: in the video, we're using the popsicle sticks, but you can easily substitute in a sheet of stiff cardboard for the popsicle sticks. (Either one works great!)Download Student Worksheet & Exercises
You now have a great grasp of Newton’s three laws and with it you understand a good deal about the way matter moves about on Earth and in space. Take a look around. Everything that moves or is moved follows Newton’s Laws.
In the next unit, we will get into Newton’s Third Law a little deeper when we discuss momentum and conservation of momentum by whacking things together *HARD*. But more on this later...
Exercises
- What is Newton’s Third Law of Motion?
- Why does the balloon stop along the string?
Click here to go to next lesson on Third Law Explained .
[/am4show]Let’s take a good look at Newton’s Laws in motion while making something that flies off in both directions. This experiment will pop a cork out of a bottle and make the cork fly go 20 to 30 feet, while the vehicle moves in the other direction!
This is an outdoor experiment. Be careful with this, as the cork comes out with a good amount of force. (Don’t point it at anyone or anything, even yourself!)
Here’s what you need to find:
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- toy car
- baking soda and vinegar (OR alka-seltzer and water)
- tape
- container with a tight-fitting lid (I don’t recommend glass containers… see if you can find a plastic one like a film canister or a mini-M&M container.)
There are two ways to do this experiment. You can either strap the bottle to the top of a toy car and use baking soda and vinegar, OR use effervescent tablets (like generic brands of alka seltzer) with this modified pop rocket (which you can strap to a toy car, or add wheels to the film canister itself by poking wooden skewers through milk jug lids for wheels and sliding the skewer through a straw to make the axle). Both work great, and you can even do both! This is an excellent demonstration in Newton’s Third Law, inertia, and how stuff works differently here than in outer space. Here’s what you do:
1. Strap the bottle to the top of the toy car or bus with the duct tape. You want the opening of the bottle to be at the back of the vehicle.
2. Put about one inch of vinegar into the bottle.
3. Shove a wad of paper towel as far into the neck of the bottle as you can. Make sure the wad is not too tight. It needs to stick into the neck of the bottle but not too tightly.
4. Pour baking soda into the neck of the bottle. Fill the bottle from the wad of paper all the way to the top of the bottle.
5. Now put the cork into the bottle fairly tightly. (Make sure the corkscrew didn’t go all the way through the cork, or you’ll have leakage issues.)
6. Now tap the whole contraption hard on the ground outside to force the wad of paper and the baking soda into the bottle.
7. Give the bottle a bit of a shake.
8. Set it down and watch. Do not stand behind the bus where the cork will shoot.
9. In 20 seconds or less, the cork should come popping off of the bottle.
What you should see is the cork firing off the bottle and going some 10 or 20 feet. The vehicle should also move forward a foot or two. This is Newton’s Third law in action. One force fired the cork in one direction. Another force, equal and opposite, moved the car in the other direction. Why did the car not go as far as the cork? The main reason is the car is far heavier then the cork. F=ma. The same force could accelerate the light cork a lot more than the heavier car.
Click here to go to next lesson on Inertia and the Second and Third Law.
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Rockets shoot skyward with massive amounts of thrust, produced by chemical reaction or air pressure. Scientists create the thrust force by shoving a lot of gas (either air itself, or the gas left over from the combustion of a propellant) out small exit nozzles.
According to the universal laws of motion, for every action, there is equal and opposite reaction. If flames shoot out of the rocket downwards, the rocket itself will soar upwards. It’s the same thing if you blow up a balloon and let it go—the air inside the balloon goes to the left, and the balloon zips off to the right (at least, initially, until the balloon neck turns into a thrust-vectored nozzle, but don’t be concerned about that just now).
A rocket has a few parts different from an airplane. One of the main differences is the absence of wings. Rockets utilize fins, which help steer the rocket, while airplanes use wings to generate lift. Rocket fins are more like the rudder of an airplane than the wings.
Another difference is the how rockets get their speed. Airplanes generate thrust from a rotating blade, whereas rockets get their movement by squeezing down a high-energy gaseous flow and squeezing it out a tiny exit hole.
If you’ve ever used a garden hose, you already know how to make the water stream out faster by placing your thumb over the end of the hose. You’re decreasing the amount of area the water has to exit the hose, but there’s still the same amount of water flowing out, so the water compensates by increasing its velocity. This is the secret to converging rocket nozzles—squeeze the flow down and out a small exit hole to increase velocity.
There comes a point, however, when you can’t get any more speed out of the gas, no matter how much you squeeze it down. This is called “choking” the flow. When you get to this point, the gas is traveling at the speed of sound (around 700 mph, or Mach 1). Scientists found that if they gradually un-squeeze the flow in this choked state, the flow speed actually continues to increase. This is how we get rockets to move at supersonic speeds or above Mach 1.
The image shown here is a real picture of an aircraft as it breaks the sound barrier. This aircraft is passing the speed at which sounds travel. The white cloud you see in the photo is related to the shock waves that are forming around the craft as it moves into supersonic speeds. Because the aircraft is moving through air, which is a gas, the gas can compress and results in a shock wave.
You can think of a shock wave as big pressure front. In this photo, the pressure is condensing water vapor in the air, hence the cloud. There are lots of things on earth that break the sound barrier – bullets and bullwhips, for example. The loud crack from a whip is the tip zipping faster than the speed of sound.
The rockets we’re about to build get their thrust by generating enough pressure and releasing that pressure very quickly. You will generate pressure both by pumping and by chemical reaction, which generates gaseous products. Let’s get started!
For this experiment, you will need:
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- film canister or other plastic container with a tight-fitting lid (like a mini-M&M container)
- alka-seltzer or generic effervescent tablets
- water
- outside area for launching
The record for these rockets is 28′ high. What do you think about that? Note – you can use anything that uses a chemical reaction… what about baking soda and vinegar? Baking powder? Lemon juice?
Important question: Does more water, tablets, or air space give you a higher flight?
Variations: Add foam fins and a foam nose (try a hobby or craft shop), hot glued into place. Foam doesn’t mind getting wet, but paper does. Put the fins on at an angle and watch the seltzer rocket spin as it flies skyward. You can also tip the rocket on its side and add wheels for a rocket car, stack rockets, for a multi-staging project, or strap three rockets together with tape and launch them at the same time! You can also try different containers using corks instead of lids.
More Variations: What other chemicals do you have around that also produces a gas during the chemical reaction? Chalk and vinegar, baking soda, baking powder, hydrogen peroxide, isopropyl alcohol, lemon juice, orange juice, and so on.
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Advanced students: Download your Pop Rockets Lab here.
Click here to go to next lesson on Forces Come in Pairs.
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The basic idea I want you to remember about Newton’s Third Law is that forces come in pairs. The wheels on a car spin, and as they do they grip the road and push the road back while at the same time the road pushes forwards on the wheel.
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Review for Forces and Newton’s Three Laws of Motion:
- Newton’s First Law is an object at rest tends to stay at rest and an object in motion tends to stay in motion unless a force acts against it.
- 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.
- Force is a push or a pull on something.
- Newton’s Second Law is F=ma or Force equals mass x acceleration. In other words, the more mass something has and/or the faster it’s accelerating, the more force it will put on whatever it hits.
- The more mass something has, the more force that’s needed to get it to accelerate. a=F/m
- Things accelerate because there is a net force acting upon them.
- Things stop accelerating (maintain a constant velocity) because the forces acting on them have equaled out.
- Newton’s Third Law states that every action has an equal and opposite reaction.
Click here to go to next lesson on Applying Newton’s Laws.
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To review, Newton’s First Law deals with objects that have balanced forces on it and predicts how they will behave. It’s sometimes called the law of inertia, and it’s the law that is responsible for helping you figure out which egg is raw or hard-boiled without having to crack it open. (If you haven’t done this, you really need to. All you have to do is set the egg spinning on the counter, then gently touch the top with a finger for a second, then release. The egg that stops dead is hard-boiled, and the one that starts spinning again in raw. Don’t know why this works? The raw egg has a liquid center that isn’t connected to the hard shell. When you stopped the shell for a split second, the innards didn’t have time to stop, and they have inertia. When you removed your finger, the liquid exerts a force on the shell and starts it spinning again. The hard-boiled egg is solid all the way through, so when you stopped the shell, the whole thing stops. Newton’s First Law in action.)
Newton’s Second Law of Motion deals with the behavior of objects that have unbalanced forces. The acceleration of an object depends on two things: mass and the net force actin on the object. As the mass of an object increases, like going from a marshmallow to a bowling ball, the acceleration decreases. Or a rocket burning through its fuel loses mass, so it accelerates and goes faster as time progresses. There’s a math equation for the second law, and it’s stated like this: F = ma, where F is the net force, m is the mass, and a is the acceleration. It’s important to note that F is the vector sum of all forces applied to the object. If you miss one or double count one of them, you’re in trouble. Also note that F is the external forces exerted on the object by other objects, not the internal forces because those cancel each other out.
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Click here to go to next lesson on Newton’s Second Law with Vector Addition.
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Newton's laws help us figure out how objects will behave when we apply forces that cause it to move. This is useful in order to be able to to land rockets on the moon, designing race cars that will accelerate faster around a turn on the highway, and so much more. But there's a problem... A lot of people can easily recite Newton's laws, but they either can't use them or understand how to apply them to the real world because they don't really know what they mean. There's evidence all around us about how the world works, but depending on what you focus on, you're going to stack different examples to support your beliefs about how the world works.
I can't tell you how many times students ask me how they can make a real light saber from Star Wars. They want to know how to make a light beam into a solid object that's tougher than steel. While I've never seen light do this in the real world, this is one of those unfortunate cases where too much media (like video games and movies) gets mixed in with how we see the world, and warps our understanding of the physical principles of the universe. There are thousands of movies and video games that use "cartoon physics" to get an action to appear on the screen a certain way, and when you watch that, it forms as a model in your mind about how the world works. If you've ever seen characters suspended in mid-air until they realize there's no ground beneath them and then they fall, or people plunging through solid walls at high speeds leaving an exact trace of their outline as they pass through it, or scaring someone which causes them to jump abnormally high in the air, you've seen this in action.
Now don't get me wrong - I love a good movie just as much as the next person, but when you're spending more time watching the world through a box, you're going to make a different model in your mind about how things behave than if you were spending time in the real world. It's not just media, though. One of the most common misconceptions we've already busted in a previous lesson is how an object needs a continuous force on it in order to continue it's motion. This one is totally not true - it's the absence of forces that makes continue its motion. One of the tasks of this physics course is to unravel these misconceptions and help you understand what's really going on by having you think for yourself, figure out what's going on, and evaluate your own thinking to see if it really makes sense.
Click here to go to next lesson on Vector Sums.
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Here's a friction experiment you can do to really see what's going on in the real world with friction:
