How are energy, time, and power related?

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A physical system that can do work on other physical systems has an amount of energy that is equal to the work that can be done. For example, a compressed spring can do work on another body when it expands. The amount of work that can be done is the energy stored in the spring. As work is the product of force and displacement in the same direction, work is expressed in terms of Newton*meter or Joule; energy also is expressed in Joules.

The rate at which a physical system does work on another physical system is referred to as power. Power is the energy expended per unit time. It is expressed in terms of Joules/second or Watts.

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