The word energy has become part of everyday life. We say that an active person is energetic. We hear a candy bar described as being full of energy. We complain about the cost of the electric energy that lights our lamps and turns our motors. We worry about some day running out of the energy stored in coal and oil. We argue about whether nuclear energy is a blessing or a curse. Exactly what is meant by energy?

In general, energy refers to an ability to accomplish change. When almost anything happens in the physical world, energy is somehow involved. But "change" is not a very precise notion, and we must be sure of exactly what we are talking about in order to go further. Our procedure will begin with the simpler idea of work and then use it to relate change and energy in the orderly way of science.