VII.

Energy

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.