Where it seems to vanish
A 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.