Where it seems to vanish
Show on a big screenA swing slows down and stops. Where did its energy go?
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- The cart starts with 96 J of motion. Heat holds 0 J.
- As it rolls, motion falls to 60 J. Heat rises to 36 J.
- Motion falls to 24 J. Heat rises to 72 J.
- The cart stops. Almost all 96 J is now heat.
The swing's energy became heat, with a little sound.
Rule
Friction is the rubbing force between two surfaces. Air resistance is the push of air against a moving thing.
Friction and air resistance turn mechanical energy mostly into heat. The mechanical energy falls. The total energy does not.
A pendulum is a weight that swings on a string. It swings a little lower each time. After some time it stops. Friction at the support and air resistance turn its energy into heat. A little becomes sound.
A 12 kg cart moves at 4 m/s. KE = ½ × 12 × 4 × 4. Now ½ × 12 = 6, and 4 × 4 = 16. So KE = 6 × 16 = 96 J. When it stops, that 96 J has mostly become thermal (heat) energy.
Rubbing warms the cart's parts and the floor. Heat moves from hot to cold. So the heat spreads into the air.
Try the lab to finish this lesson.