In a pendulum with air resistance, which statement best describes energy transformation?

• A. Mechanical energy remains constant.
• B. Thermal energy decreases as the pendulum slows.
• C. Mechanical energy gradually converts to thermal energy due to friction.
• D. Kinetic energy increases while potential energy stays constant.

Answer: (C)

When a pendulum moves through air, there are non-conservative forces at work (air drag and pivot friction) that oppose the motion and do negative work on the pendulum. This removes mechanical energy—the sum of kinetic and potential energy—from the swinging system. That energy isn’t lost from the universe; it is transformed into thermal energy, warming the air and the pendulum itself, so the surroundings’ thermal energy increases over time. Because of this dissipation, the total mechanical energy of the pendulum drops with each swing, and the motion gradually decays in amplitude.

At the bottom of the swing the kinetic energy is high and the potential energy is low, while near the top the potential energy is higher but the overall mechanical energy is lower than in a frictionless case due to the ongoing energy loss. Therefore, the best description is that mechanical energy gradually converts to thermal energy due to friction, which is why the pendulum slows down and eventually stops.