Energy changes form
You eat, run, and get tired. The energy in your food became something else.
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Step 1 of 4
- A ball is dropped from 2 m. At the top, it is still. All its energy is PE.
- Partway down, the speed is 4 m/s. Some PE has become KE.
- Near the ground, the speed is 6 m/s. Most energy is KE.
- PE + KE stays the same all the way down.
In your body, food energy became motion and heat. Most became heat.
Rule
Energy changes from one form to another. It is not created or destroyed. If only gravity acts, PE + KE stays the same.
- Mechanical energy — KE + PE.
Energy has other forms too: heat, light, sound, electrical, chemical and nuclear.
- A bulb turns electrical energy into light and heat.
- Food gives chemical energy to your muscles.
- A microphone turns sound energy into electrical energy.
Now take a 1 kg ball that falls from 20 m. At the top, the ball is still, so KE = 0. PE = 1 × 10 × 20 = 200 J. At the ground, the height is 0, so PE = 0. PE + KE stays 200 J, so KE = 200 J.