The rule of conservation of energy states energy in a closed system is:

• A. Conserved; cannot be created or destroyed, only transformed.
• B. Energy can be created in some systems.
• C. Energy can be destroyed in some systems.
• D. Energy is neither created nor destroyed nor transformed.

Answer: (A)

Energy in a closed system is conserved: it cannot be created or destroyed, only transformed. This means energy can change from one form to another—like potential energy turning into kinetic energy, or into heat—but the total amount stays the same. For example, a pendulum swaps between high potential energy and high kinetic energy as it swings, and the sum of those energies remains constant when air resistance and friction are negligible. If friction is present, some energy becomes heat in the surroundings; still, when you account for that thermal energy, the overall energy balance holds. The statements that energy can be created or destroyed, or that it’s neither created, destroyed, nor transformed, don’t match how energy behaves in real processes.