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An extremely important concept that has been developed in physics is that of the work done on a body by the action of some external agent which exerts a force on this body and produces motion. For example, whenever someone lifts a body, he does work by exerting a force upward on it and moving it upward. Whenever a steam locomotive pulls a train, a series of processes takes place in the steam engine of the locomotive which enables it to exert a force on the train and move it in the direction of the force. The term work, as used in physics, is a technical term. Whenever work is done by an external agent on a body, the work done is the product of the force which acts on the body and the distance through which the body moves while the force is acting on it, provided that the force and the distance through which the body moves are parallel to each other.
In the initial development of the concept of work, we shall restrict our discussion to work done by a constant force. We shall later (Section 7-8) remove this restriction and treat the more general case of work done by a variable force.