A student observes a small-angle pendulum swinging back and forth. Which of the following best explains why the bob’s speed is zero at each end of its swing?
(a) The net force on the bob is zero at the endpoints because the tension in the string exactly balances gravity, so the bob cannot keep moving.
(b) The bob’s momentum must reverse direction without an external impulse, so its momentum—and therefore its speed—must pass through zero at the endpoints.
(c) All of the system’s mechanical energy has been converted to gravitational potential energy, leaving no kinetic energy available for motion.
(d) Because the bob’s centripetal acceleration is zero at the endpoints, its tangential speed must also be zero.