Whenever a force is exerted on a moving object, we say that work is done by the force or by the agent exerting the force. For example, if you exert a force on an object to life it, you are doing work. If the force of friction slows down a moving object, work is done by the force of friction. If an unbalanced force accelerates an object, work is done by he unbalanced force.

We measure the amount of work that is done by multiplying the force exerted and the distance the object moves in the direction of the force (or, more precisely, the magnitude of the component of the object's displacement parallel to the direction of the force).

W = Fd

Energy is defined as the ability to do work. We measure how much energy an object has by the amount of work it can do.

An object can have energy, the ability to do work, (a) because it is moving, or (b) because of its position in a force field, or because of its temperature. We call these three kinds of energy kinetic energy, potential energy, and the heat energy respertively.

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