Energy and work are two very central, closely related concepts in the physics of motion. In fact, energy can itself be defined as the capacity to do work. Work, in turn, is defined as the force required to move an object some distance. Therefore, a transfer of energy occurs with work as one body moves another against an opposing force, such as gravity or friction.

There are many forms of energy. Heat energy, light energy, and potential energy are examples. With regard to work, however, kinetic energy is central. Kinetic energy is the energy of motion. Because all moving matter has mass and velocity, the kinetic energy of moving objects can be expressed in terms of these. Kinetic energy, then, is defined as one-half the product of the mass of an object and the square of its velocity. Therefore, the greater the mass of...