Potential and Kinetic Energy and Energy Conservation
You are at the top of the first hill on a roller coaster. The car clicks, pauses, and then drops. Where does the thrill of that speed come from? And when the ride ends and the car stops, where did all that energy go?
Kinetic and potential energy
Energy is the ability to cause change. It can make things move, heat up, or light up. We measure it in joules (J).
Energy comes in two big types.
There are three kinds of potential energy to know:
| Type | What stores it | Example |
|---|---|---|
| Gravitational potential | An object's height above the ground | A book on a high shelf, a skier at the top of a hill |
| Elastic potential | A stretched or squeezed object | A stretched rubber band, a compressed spring |
| Chemical potential | Bonds in food, fuel, and batteries | A bowl of pasta, gasoline, a battery |
Why does height store energy? Gravity pulls the object down. If you lift it up, you did work against gravity, and that energy is stored. Let go, and gravity turns it into motion.
Define kinetic energy and gravitational potential energy. Give one everyday example of each.
Define kinetic energy and gravitational potential energy. Give one everyday example of each.
Classify each as mainly gravitational potential, elastic potential, chemical potential, or kinetic energy: (a) a stretched rubber band, (b) a bowl of pasta, (c) a book…
Classify each as mainly gravitational potential, elastic potential, chemical potential, or kinetic energy: (a) a stretched rubber band, (b) a bowl of pasta, (c) a book on a high shelf, (d) a cyclist riding down a street.
Energy changes form
Energy rarely stays in one form. An energy transformation is a change from one form to another. An energy transfer is energy moving from one object or place to another, often staying the same form.
Examples:
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A battery lights a bulb: chemical potential → electrical → light + thermal.
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You eat pasta and sprint: chemical potential (food) → kinetic + thermal.
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A cyclist coasts down a street: gravitational potential → kinetic.
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Hot soup warms a spoon: thermal energy is transferred from soup to spoon.
Energy also moves through living things. In a food web, the Sun's light energy is transformed by plants during photosynthesis into chemical potential energy in sugars. When an animal eats the plant, that chemical energy is transferred to the animal.
Not all the energy an animal eats is stored in its body. Most is used for moving, growing, and staying warm. It leaves as thermal energy (heat) into the environment.
Trace the energy transformations in a simple circuit: a battery is connected to a light bulb. Name the type of energy at each stage (the battery, the moving charges, and…
Trace the energy transformations in a simple circuit: a battery is connected to a light bulb. Name the type of energy at each stage (the battery, the moving charges, and the light and heat given off by the bulb). Explain why the total amount of energy stays the same.
Aaliyah draws a food web in which grass is eaten by a grasshopper, the grasshopper is eaten by a frog, and the frog is eaten by a hawk. Describe how energy from the Sun…
Aaliyah draws a food web in which grass is eaten by a grasshopper, the grasshopper is eaten by a frog, and the frog is eaten by a hawk. Describe how energy from the Sun is transformed and transferred along this food web. Explain what happens to the energy that is not stored in each organism's body.
Chloe compares three systems: a plant doing photosynthesis, a wind-up toy, and a swing at an amusement park. For each system, identify one form of potential energy and…
Chloe compares three systems: a plant doing photosynthesis, a wind-up toy, and a swing at an amusement park. For each system, identify one form of potential energy and one transformation that occurs. Then compare how the gravitational, elastic, and chemical potential energies are alike and different from kinetic energy, and explain how energy is conserved in at least one of the systems.
The law of conservation of energy
So when a ball rolls to a stop, its energy is not "used up." Friction between the ball and the floor changes kinetic energy into thermal energy. The floor and ball get a tiny bit warmer, and a little sound is made. The energy is still there, just spread out where it is hard to use.
Now think of the roller coaster. At the top of the hill, the car stops for a moment: all gravitational potential energy, almost no kinetic. As it drops, height shrinks, so potential energy goes down, while speed grows, so kinetic energy goes up. At the bottom, kinetic energy is greatest. If friction is very small, the sum of the two stays almost constant.
A swing trades gravitational potential energy and kinetic energy again and again. At the highest points it has the most potential energy, and at the lowest point it has the most kinetic energy. The total stays almost the same, and friction with the air slowly turns some of it into thermal energy.
This also answers why the ball's kinetic energy at the bottom is less than 10 J: some of the starting energy became thermal energy, which does not make the ball move.
Sofia says, "Energy is used up when a ball stops rolling." Is she correct? State the law of conservation of energy and say where the ball's energy goes.
Sofia says, "Energy is used up when a ball stops rolling." Is she correct? State the law of conservation of energy and say where the ball's energy goes.
A roller coaster car is pulled to the top of the first hill and stops for a moment. Then it rolls down. Describe how its gravitational potential energy and kinetic…
A roller coaster car is pulled to the top of the first hill and stops for a moment. Then it rolls down. Describe how its gravitational potential energy and kinetic energy change as it moves from the top to the bottom of the hill. Say what happens to the total energy if friction is very small.
A $0.5\ \text{kg}$ ball is dropped from the top of a ramp. Its energy is measured at three points, in joules (J): | Point | Gravitational potential energy (J) | Kinetic…
A $0.5\ \text{kg}$ ball is dropped from the top of a ramp. Its energy is measured at three points, in joules (J):
| Point | Gravitational potential energy (J) | Kinetic energy (J) | Thermal energy from friction (J) |
|---|---|---|---|
| Top | 10 | 0 | 0 |
| Middle | 6 | 3 | ? |
| Bottom | 0 | 8 | ? |
(a) Calculate the missing thermal energy values. (b) Calculate the total energy at each point and use your results to explain whether energy is conserved. (c) Explain why the ball's kinetic energy at the bottom is less than $10\ \text{J}$.
Back to the start: the roller coaster's speed comes from gravitational potential energy stored when the car was pulled up the hill. When the ride ends, brakes turn its kinetic energy into thermal energy. Nothing disappears.
Summary
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Kinetic energy is energy of motion. Potential energy is stored energy: gravitational (height), elastic (stretched or squeezed), chemical (food, fuel, batteries).
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Energy is measured in joules (J).
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A transformation changes the form of energy; a transfer moves energy from one object to another.
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In food webs, light energy becomes chemical energy in plants and moves up the web, with heat lost at each step.
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Law of conservation of energy: energy is never created or destroyed, so the total stays the same. Friction turns some energy into thermal energy.