Question

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?

In a closed system, kinetic energy is always less than potential energy.
Some potential energy was converted into thermal energy due to friction.
The acceleration due to gravity became smaller as the box slid down the ramp.
In a closed system, total energy is constantly being lost to outside objects.

Answer 1

Some potential energy was converted into thermal energy due to friction.

Explanation: If friction (or other non‑conservative forces like air resistance) acts while the box slides, part of the initial gravitational potential energy mgh is turned into heat, sound, or internal energy, so the kinetic energy at the bottom is less: mgh = KE_bottom + energy lost to friction. The other statements are incorrect: gravity didn't change appreciably, and in a closed system energy isn't "constantly lost" — it's conserved overall, just converted into nonmechanical forms.