Question

The image shows a pendulum that is released from rest at point A. Shari tells her friend that no energy transformation occurs as the pendulum swings between point B and point D.

An illustration of a pendulum at 5 different positions. The central position, the equilibrium position is labeled C. The two positions on the left are labeled A and B respectively. The two positions on the right are labeled D and E respectively. All positions are equally spaced.

What is the best response that Shari’s friend could make?

She could agree with Shari because the pendulum is at the same height and would have the same gravitational potential energy at both positions.
She could agree with Shari because the pendulum has the same amount of mechanical energy throughout its swing.
She could disagree with Shari because the pendulum converts kinetic energy into gravitational potential energy, then back into kinetic energy.
She could disagree with Shari because the pendulum converts gravitational potential energy into kinetic energy, then back into gravitational potential energy.

Answer 1

The best response is the fourth option:

She could disagree with Shari because the pendulum converts gravitational potential energy into kinetic energy, then back into gravitational potential energy.

Explanation: As the pendulum falls from A toward C (through B) potential energy is converted to kinetic energy (maximum at C). As it rises toward D (and E) kinetic energy is converted back into gravitational potential energy. Although B and D are at the same height (same potential), energy is being exchanged between kinetic and potential during the motion.