If energy is conserved, why do bouncing balls, pendulums, and
other cyclic systems eventually stop moving? The energy doesn’t
disappear but rather it is transferred to the surroundings, where
it is unavailable to keep the cycle going. The result is that
most objects moving in cycles eventually move less and less and
then finally stop.
If we consider the bouncing ball striking a table, some energy
is transferred to the table causing it to vibrate. This vibration
moves the air, creating a sound. The mechanical vibration of the
table also transfers energy to the floor of the room through the
table legs and heat is produced and escapes to the room as the
table deforms slightly at the point of impact. Also, as the ball
falls, some energy is used to overcome the effects of its motion
through the air and additional heat is produced within the ball
as it stretches and deforms at the table surface.
All these transformations allow energy to escape from the system,
resulting in a net loss with each repetition of the cycle. We
take these losses to the surroundings for granted and if we want
to keep a ball bouncing, we give it a little push downward at
the beginning of each bounce.
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