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. What has happened to the energy?
The energy goes into the ground and, as a result, the orbit of
the earth about the Sun is slightly changed.
The energy goes to producing sound and to heating the ground,
rock, and surrounding air.
The rock keeps the energy inside it (saving it for later
use).
It is lost forever. Energy does not have to be conserved.
It is transformed back into gravitational potential energy