Answer: Friction.
- A Gravity
- B Spring force
- C Friction
- D Electrostatic force
Correct answer: C. Friction
Explanation: Friction is non-conservative: work done by friction depends on the path taken, and energy is lost as heat.
As a pendulum swings, total mechanical energy (KE+PE) stays constant: at the highest points A and C, all energy is potential (zero speed); at the lowest point B, all of that energy has converted into kinetic energy (maximum speed).
Concept context
Work-energy theorem, potential energy, conservation of energy, collisions.