Which statement best describes energy conservation in a closed system?

• A. Energy cannot be transformed between kinetic and potential.
• B. Total energy can increase over time.
• C. Energy is created when kinetic energy increases.
• D. Total energy remains constant; it can shift between kinetic and potential.

Answer: (D)

The main idea being tested is that in a closed system, energy cannot be created or destroyed; it just moves between forms. In a system with gravity and negligible friction, the total mechanical energy (the sum of kinetic and potential energy) stays the same as the object moves. As the object climbs, potential energy increases while kinetic energy decreases; as it descends, kinetic energy increases and potential energy decreases. The total energy remains constant because no energy enters or leaves the system.

So the best statement is that the total energy stays constant and can shift between kinetic and potential. The total amount of energy doesn’t rise over time, and energy isn’t produced or created when speed changes; rather, it is converted between forms. This is why a pendulum or roller-coaster car can speed up or slow down while the overall energy stays the same, assuming no friction losses.