Asimov wrote:
If I can manage to roll the boulder up at the top or at the bottom of
the cliff, I will find it weighs about the same. The boulder only
picks up energy as it is accelerating towards the foot of the cliff.

Energy cannot be created or destroyed. The kinetic energy gained
as it accelerates is exactly balanced by the loss in potential
energy that it had at the top of the cliff.

Consider a pendulum. When it stops at the limit of its swing,
it possesses potential energy. When it is moving at its fastest
at the bottom of its swing, it possesses kinetic energy. When
it finishes the swing to the other limit, it possesses potential
energy. Neglecting losses, potential energy at the top of the
swing is converted into an equal magnitude of kinetic energy at
the bottom of the swing and back to potential energy at the top
of the swing.
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73, Cecil http://www.qsl.net/w5dxp