Jugami › Physical Science › Energy
It never gets used up. It just changes form.
A roller coaster climbs slowly, then screams down the hill. A phone charges overnight and runs all day. A sandwich powers your whole afternoon. Each one runs on energy that's stored, moved and changed into something else. On this page you'll learn the main kinds of energy, how to measure it, and the big rule that energy is never created or destroyed.
Energy is the ability to do work or cause change. Anything that moves, heats up, lights up, or makes a sound is using energy.
Scientists measure energy in joules (symbol J). One joule is the energy used when a force of 1 newton pushes something 1 meter. Lifting a small apple about 1 meter off a table takes roughly 1 joule.
Kinetic energy is the energy of motion. A rolling bowling ball, a flying baseball, and the wind all have it. The faster and heavier something is, the more kinetic energy it has.
Potential energy is stored energy, waiting to be used. There are several kinds:
| Form | What it is | Example |
|---|---|---|
| Thermal | The motion of the tiny particles in a substance; we feel it as heat | A hot cup of cocoa |
| Light (radiant) | Energy that travels as light and other electromagnetic waves | Sunlight, a flashlight |
| Sound | Vibrations moving through air, water, or solids | A drum, a voice |
| Electrical | Energy carried by moving electric charges | Lightning, current in a wire |
| Nuclear | Energy stored in the centers (nuclei) of atoms | The Sun, nuclear power plants |
| Chemical | Energy stored in chemical bonds | Food, batteries, fuel |
Energy constantly changes from one form to another. That's called an energy transformation. A toaster turns electrical energy into thermal energy. Your muscles turn chemical energy from food into kinetic energy.
The law of conservation of energy says energy cannot be created or destroyed. It can only change form or move from one object to another. If you add up all the energy before and after, the total is the same.
Work
W = F × d
In science, work is done when a force moves something. Work is measured in joules, because doing work transfers energy. If the object doesn't move, no work is done, no matter how hard you push.
Worked example: You push a wagon with 10 N for 3 m. W = 10 N × 3 m = 30 J.
Worked example: You lift a 20 N backpack 1.5 m onto a shelf. W = 20 N × 1.5 m = 30 J.
Kinetic energy
KE = ½ m v²
Mass (m) is in kilograms and speed (v) in meters per second. Because speed is squared, doubling the speed makes the kinetic energy four times bigger. That's why fast cars need so much more distance to stop.
Worked example: A 2 kg ball rolls at 3 m/s. KE = ½ × 2 kg × (3 m/s)² = ½ × 2 × 9 = 9 J.
Gravitational potential energy
PE = m g h
Near Earth's surface, g is about 9.8 m/s². Height (h) is in meters.
Worked example: A 2 kg rock sits on a 5 m ledge. PE = 2 kg × 9.8 m/s² × 5 m = 98 J. If it falls, that 98 J turns into kinetic energy (minus a little lost to air resistance).
No machine turns all of its energy into the form we want. Some always ends up as thermal energy, usually from friction. We call it "wasted" because it isn't useful, but it isn't destroyed; it just spreads out into the surroundings.
Efficiency is how much of the energy put in comes out as useful energy:
efficiency = useful energy out ÷ total energy in × 100%
Worked example (made-up numbers): Imagine a bulb that uses 100 J of electrical energy and gives out 20 J of light. Its efficiency is 20 ÷ 100 × 100% = 20%. The other 80 J becomes heat. (Real old-style incandescent bulbs turn only about 5% into light; LEDs waste much less.)
Set the hill's height and the car's mass. The tool works out the potential energy at the top (PE = m g h), and the speed at the bottom if no energy were lost to friction (all PE becomes KE).
PE at top = 98,000 J · speed at bottom ≈ 19.8 m/s
At the top: PE 100%, KE 0%. The total stays the same all the way down.
Notice: the mass changes the energy, but not the speed at the bottom.
Ask an adult before you start. Work on the floor, away from breakable things, and keep anything that bounces, rolls or stretches away from people's eyes.
You need: a rubber or tennis ball, a tape measure.
Drop the ball from 1 meter onto a hard floor. How high does it bounce? Try three times.
Why: It never bounces back to 1 meter. Some energy became sound (the thump) and heat in the ball and floor.
You need: one wide rubber band.
Touch the band to your upper lip or the inside of your wrist. Quickly stretch it a few times, then touch it again. Don't aim or snap it.
Safety: Use a band that isn't cracked or worn, stretch it gently, and keep it away from your eyes. If you have a latex allergy, use a latex-free band or hold it to the inside of your wrist instead.
Why: Stretching does work on the band, and some of that energy becomes thermal energy, so it feels a little warmer.
You need: a book stack, a board or stiff cardboard, a toy car or marble, a tape measure.
Release the car from a ramp 1 book high, then 2, then 3. Measure how far it rolls each time.
Why: More height means more potential energy, which becomes more kinetic energy at the bottom.
You need: just your hands.
Rub your palms together fast for 10 seconds. Then try it with a drop of lotion.
Why: Friction turns the kinetic energy of your hands into heat. Lotion lowers friction, so less heat is made.
Credits: All illustrations on this page are original, hand-coded SVG by Galileo. The share image was drawn in code.
