of Conservation of energy:
Energy can neither be created nor destroyed but it can
transformed from one form to another.
Think about the wound-up spring that is accelerating a
clockwork toy. Strain energy changes into kinetic energy and
heat (the heat energy passes into the toy and makes it slightly
warmer). The total amount of kinetic and heat energy produces
equals the amount of strain energy that disappears.
Energy can be measured , and the unit to measure all forms of
energy is the JOULE. If the spring loses 20 joules of strain
energy , the 20 joules of energy must appear in other
forms-Kinetic and heat in this case. There can be no overall
loss of energy. Energy cannot be destroyed.