Conservation of Energy: Tracking the Total

0:00A swinging pendulum slows down and stops. Where does its energy go?

0:06A pendulum is a heavy ball, the bob, on a string hanging from a pivot.

0:12Treat the pendulum and the air around it as a closed system: no energy enters or leaves.

0:19The bob starts with 10 joules of gravitational potential energy, stored because of its height above the lowest point.

0:28With no friction yet, the bob falls and speeds up, gaining kinetic energy: the energy of motion.

0:35At the lowest point, all 10 joules are kinetic, and the bob moves fastest.

0:41As it rises, it slows down, and kinetic energy turns back into potential energy.

0:48Add them up at any moment: the total energy is always 10 joules.

0:54This is the law of conservation of energy: energy changes form, but is never created or destroyed.

1:02But a real pendulum swings lower each time. Is energy used up?

1:08No. Friction at the pivot and air resistance on the bob cause heating.

1:13Thermal energy is the energy of moving particles. The pivot and air gain some and get slightly warmer.

1:21At the top, 7 joules of potential plus 3 joules of thermal energy still make 10 joules.

1:29Swing by swing, more energy becomes thermal. When it stops, all 10 joules are thermal energy.

1:37The energy is not lost. It is spread out, and far less useful.

1:42Energy changes form, but in a closed system the total always stays the same.