# Work done per time is power

Work is a measure of energy. If I lift a book of mass m = 1 kg upward a distance of L = 1 m against the force of gravity, I need to produce

a total of 9.8 Joules of work. Fine.

But it takes the same amount of energy whether I lift the book quickly, in just 1 second, or if I very very sl-o-o-o-o-wly raise the book over 20 seconds. The same number of Joules, yes .... but something is different.

The difference there is in the RATE at which I do work. We have a name for the rate at which energy is used: power.

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Q:  What are the units of power?

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Let's see how much power a person can produce. We'll run an experiment:

• a volunteer of mass m
• will run up a flight or two of stairs, a vertical distance H
• over a period of time T

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Q:  How much work will the volunteer do?

Q:  How much power will the volunteer produce?

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After we've made the measurements and computed the power of our student volunteer, we can compare it to the power produced by a horse. You may have heard of a "horsepower" -- it is based on the amount of work that a horse could do over an entire day. In metric terms, one horsepower is about 746 Watts.

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Is our volunteer equal to one horse?

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