The quantity called energy
ties together all branches of physics. In the field of mechanics, energy
must be provided to do work; work is defined as the product of force and
the distance an object moves in the direction of the force. When a force
is exerted on an object but the force does not cause the object to move,
no work is done. Energy and work are both measured in the same units—ergs,
joules, or foot-pounds, for example.
If work is done lifting an object to a greater
height, energy has been stored in the form of gravitational potential
energy. Many other forms of energy exist: electric and magnetic potential
energy; kinetic energy; energy stored in stretched springs, compressed
gases, or molecular bonds; thermal energy; and mass itself. In all
transformations from one kind of energy to another, the total energy is
conserved. For instance, if work is done on a rubber ball to raise it, its
gravitational potential energy is increased. If the ball is then dropped,
the gravitational potential energy is transformed to kinetic energy. When
the ball hits the ground, it becomes distorted and thereby creates
friction between the molecules of the ball material. This friction is
transformed into heat, or thermal energy.
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