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A ball is dropped on a horizontal floor, bounces repeatedly to decreasing heights, and finally comes to rest. When this process is filmed and played in reverse, the reverse process is not observed in nature. Which law of physics best explains why the reverse process is impossible?

1. Kelvin-Planck statement of the second law of thermodynamics
2. Clausius statement of the second law of thermodynamics
3. Carnot's theorem
4. First law of thermodynamics

Correct answer: Kelvin-Planck statement of the second law of thermodynamics

Solution

During bouncing, ordered kinetic energy converts irreversibly to disordered thermal energy. The reverse would require converting heat from the floor completely into mechanical energy with no other effect — exactly what the Kelvin-Planck statement forbids.

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