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| Power |
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| A contractor wishes to lift a load of bricks from the sidewalk to the top of a building by means of a winch. Since we know the expression for work, we can calculate work to be done by the force applied by the winch to lift the load. We are interested in the rate at which it is done, that is, will the job take 5 minutes or a week. |
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| The time rate at which work is done by a force is said to be the power due to the force. If an amount of work 'W' is done in an amount of time 'Dt' by a force, the average power due to the force during that time interval is |
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| The instantaneous power 'P' is the instantaneous rate of doing work, which can be written as |
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| The SI unit of power is joule per second. It is called 'watt(W)', after the scientist, James Watt. |
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| In the case of electrical energy, the common unit used is the kilowatt hour. |
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