## Item NG018003: The thermal energy of a ball and a track increases the entire time the ball is rolling along the track because both the gravitational potential energy and the motion energy of the ball are converted into thermal energy.

A ball, starting from rest at Position 1, rolls along a curved track toward Position 5.  The ball speeds up as it rolls from Position 1 to Position 3, and it slows down as it rolls from Position 3 toward Position 5.  Just before it reaches Position 5, it stops and rolls back down the track.  As the ball rolls back and forth along the curved track, the ball and the track get a little warmer.

When does the thermal energy of the ball and track increase and why?

1. Only when the ball rolls from Position 1 to Position 3 because the gravitational potential energy of the ball is converted into thermal energy
2. Only when the ball rolls from Position 3 toward Position 5 because the motion energy (kinetic energy) of the ball is converted into thermal energy
3. The thermal energy increases the entire time the ball is rolling along the track because both the gravitational potential energy and motion energy (kinetic energy) of the ball are converted into thermal energy.
4. The thermal energy increases the entire time the ball is rolling along the track, but the increase does not come from a change in gravitational potential energy or motion energy (kinetic energy) because motion energy and gravitational potential energy cannot be converted into thermal energy.