The term energy is closely associated with the concepts of force and motion.
Naturally moving matter, such as the wind or moving water, exerts forces. You
have felt these forces if you have ever tried to walk against a strong wind
or stand in one place in a stream of rapidly moving water. The motion and forces
of moving air and moving water are used as energy sources (Figure 4.1). The
wind is an energy source as it moves the blades of a windmill performing useful
work. Moving water is an energy source as it forces the blades of a water turbine
to spin, turning an electric generator. Thus, moving matter exerts a force,
on objects in its path, and objects moved by the force can also be used as an
energy source. Matter does not have to be moving to supply energy; matter contains energy.
Food supplied the energy for the muscular exertion of the humans and animals
that accomplished most of the work before this century. Today, machines do the
work that was formerly accomplished by muscular exertion. Machines also use
the energy contained in matter. They use gasoline, for example, as they supply
the forces and motion to accomplish work. Moving matter and matter that contains energy can be used as energy sources
to perform work. The concepts of work and energy and the relationship to matter
are the topics of this chapter. You will learn how energy flows in and out of
your surroundings as well as a broad, conceptual view of energy that will be
developed more fully throughout the course. | 
2002 McGraw-Hill Higher Education