Answer: Centripetal force.
- A Gravity
- B Friction
- C Centripetal force
- D Applied tangential force
Correct answer: C. Centripetal force
Explanation: Centripetal force acts perpendicular to velocity (toward center), so it does zero work on the object.
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.