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Mathematically, power is a rate quantity - a time-based quantity. It is the rate at which a force does work upon an object. Power is calculated from knowledge of the work done upon an object (W) and the time (t) required to do this work. The formula is
P = W / t
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It is often the case that the units of a quantity can be determined from the formula for the quantity. In the case of power, the units would be a unit of work divided by a unit of time. This would be a Joule / second, abbreviated J / s. The unit Joule / second is defined to be equivalent to a Watt. Thus, Watt is the standard unit of power in the metric system.
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