Any energy transfer involves a source, whose energy is reduced; a means of transferring the energy; and an energy receiver, whose energy is increased. It’s convenient to use a diagram to keep track of the source, the transfer, and the receiver (see pp. 61–62).

For example, if a moving pool ball collides with another ball it can transfer all or part of its kinetic energy to the other ball. Transfer of energy by this kind of mechanical interaction is called work. The moving ball does work on the stationary ball and its kinetic energy is reduced. The kinetic energy of the stationary ball is increased by the work done on it.

When a slingshot does work on the stone, its stored elastic energy decreases and the kinetic energy of the stone increases.

When you throw a ball your stored chemical energy is reduced, work is done on the ball, and the ball’s kinetic energy is increased.

Other methods of transferring energy that do not involve work will be discussed later.