What happens to gravitational potential energy when a rock tumbles downhill and accelerates?

• A. Gravitational potential energy is converted into kinetic energy
• B. It becomes thermal energy
• C. It disappears
• D. It becomes chemical energy

Answer: (A)

The main idea is energy transformation as the rock moves under gravity. Gravitational potential energy depends on height, so as the rock tumbles downhill its height—and thus its gravitational potential energy—decreases. The energy gravity does as the rock moves downward shows up mainly as an increase in kinetic energy, which is why the rock speeds up (it’s accelerating). This is a direct expression of the work-energy idea: gravity does positive work on the rock, converting gravitational potential energy into kinetic energy. If there’s any friction, some energy can become heat, but the primary change is that gravitational potential energy becomes kinetic energy.