When a rock is held above the ground, we say it has some potential energy. When we let it go, it falls and we say the potential energy is converted to kinetic energy. Finally, the rock hits the ground (and stays there). What has happened to the energy?

Energy converted to heat and sound. Deformation of soil or ground may also take up some energy.

Explanation:

When the rock hits the ground, a part of the energy is converted to heat (thermal energy) and sound. If the ground is soft, it will be compressed by the rock when it lands. This deformation of the ground will take up some of the energy.

We can think of kinetic energy being transferred to the ground in the following way:

1. Ball hits the ground, and transfers kinetic energy to lots of soil particles.

2. Soil particles move a bit and transfer the energy to soil particles ahead of them.

3. Kinetic energy is transferred to a very large number of soil particles, and they move so little that we consider this vibration motion.

4. This small vibration motion is now considered as internal energy or 'heat'.

conduction, convection, radiation