A box is released from rest and slides down a ramp in a closed system. Even though the box speeds up, its kinetic energy at the bottom of the ramp is less than the potential energy that it had at the top of the ramp. What best explains why the box’s final kinetic energy is less than its initial potential energy?

As the box slides, friction converts some of its gravitational potential energy into thermal energy. The box speeds up, so it gains kinetic energy, but not all the initial potential energy becomes kinetic energy; some becomes thermal energy. Thus, choice A, “Some potential energy was converted into thermal energy due to friction,” best explains the difference.

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