Final answer:
A boulder on a mountain has potential energy that can convert to kinetic energy if it rolls down, but some energy is lost as thermal energy due to friction.
Step-by-step explanation:
A boulder sitting on a mountain has potential energy due to its elevated position above the ground. Potential energy is the energy that an object has because of its position in a force field, in this case, the gravitational field of the Earth. If the boulder were to roll down the mountainside, this potential energy would be converted into kinetic energy, which is the energy of motion. The transformation of energy does not stop there; as the boulder rolls up the opposite slope, kinetic energy is then converted back into potential energy. However, not all of the energy can be converted back because some of it is lost as thermal energy due to friction, resulting in the boulder stopping partway up the ridge.