State the principle of conservation of energy in a closed system. Provide a simple example.

• A. Energy is created when motion occurs.
• B. Total energy can increase due to external work.
• C. Energy cannot be transformed between forms.
• D. In a closed system, total energy remains constant; it can transfer between kinetic and potential.

Answer: (D)

In a closed system, energy cannot be created or destroyed; the total amount stays the same. It can move between objects and change forms, such as from potential energy to kinetic energy and back, but the sum of all forms remains constant. A simple example is a pendulum swinging in air with negligible friction: at the highest point, energy is mostly potential with little kinetic; as it swings downward, potential energy converts to kinetic energy, so the kinetic energy increases while the potential decreases, yet the total energy stays the same (ignoring air resistance). If there were external work or friction, that could change the total energy, but in a true closed system with no external input, the total energy is constant.