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.

As the box slides down, some of its gravitational potential energy becomes kinetic energy, so the box speeds up. Friction also converts some of that potential energy into thermal energy, leaving the box with less kinetic energy than the potential energy it started with. Thus, “Some potential energy was converted into thermal energy due to friction” is correct; total energy is still conserved in the closed system.

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