In physics, '''power''' refers to the rate of [[Work|transfer of energy]] respect to [[time]].<ref>Serway and Beichner, ''Physics For Scientists and Engineers'', Fifth Edition</ref>
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Mathematically, average power is expressed as:
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P = ''W'' / Δt
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Therefore, power at any instant can be determined by the equation:
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P = lim<sub>Δt->0</sub> ''W'' / Δt = dW / dt
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Since d''W'' = [[Force|F]] d[[distance|x]] and [[Velocity|v]] = dx / dt follows that:
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P = F · dx / dt = F · v
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The usual units for power are [[Watts]] ( W = [[Joule|J]]/[[seconds|s]] ) and [[horsepower]] ( hp = 746 W )