Question

A book is falling from the top shelf of a bookcase. Which statement correctly describes the change in the energy of the book?
The potential energy and kinetic energy remain the same.
The potential energy decreases as kinetic energy increases.
The kinetic energy decreases as potential energy increases.
The kinetic energy remains at zero, but potential energy increases.

Answer 1

The potential energy decreases as kinetic energy increases.

As the book falls, gravitational potential energy is converted into kinetic energy (neglecting air resistance), so the book speeds up while its potential energy relative to the shelf decreases. If nonconservative forces (air resistance, friction) act, some energy is also dissipated as heat.