Which principle states that energy cannot be created or destroyed?

• A. Kinetic Energy
• B. Momentum
• C. Elastic Collision
• D. Conservation of Energy

Answer: (D)

Energy cannot be created or destroyed; in any system, the total amount of energy stays the same even as it changes from one form to another. This is the idea behind conservation of energy: kinetic energy, potential energy, thermal energy, and other forms can trade off, but their total sum remains constant in an isolated system.

That’s why a swinging pendulum or a ball rising and falling demonstrates it: as kinetic energy decreases and potential energy increases, the total energy remains unchanged (ignoring air resistance and friction). If non-conservative forces like friction are present, mechanical energy isn’t constant, but the overall energy of the system, including heat, still remains constant.

Kinetic energy is just one form of energy and can vary a lot during motion. Momentum is a different quantity that is conserved in collisions, but it doesn’t state the general rule about all energy. An elastic collision is a special case where kinetic energy is also conserved, but not all processes are elastic. The principle that best captures the general rule is conservation of energy.