Energy is the ability to do work—to exert a force over a distance. This ability may reside in many different places--in the muscles of a horse or a human body, in coal that can be burned, in the electrical energy distributed from the power company, in the light of the sun, in a nuclear reactor, etc. But while man has known how to use energy since time immemorial, it is only in about the last 200 years that it has been understood that energy can be converted from one form into another.

There are two kinds of energy a simple object can have. Kinetic energy is energy due to motion, such as the energy of a car moving down the highway. Potential energy is energy gained from moving an object from one place to another, such as a ball picked from the ground and held at arm's...