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**The rate at which work is done.**

Objective: To define and apply the concepts of power and mechanical efficiency. Power The rate at which work is done.

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**The power is the work done divided by the time it takes to do the work.**

Units: J/s = Watt 1 horsepower =746 Watts 1000 Watts = 1 kW Kilowatt-hours (kwh) measure energy.

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**Leaving a 60W light bulb on for 24 hours will cost about 15 cents.**

SRP charges about 10 cents for 1kwh. How much would it cost to leave a 60W light bulb on for a full day? Leaving a 60W light bulb on for 24 hours will cost about 15 cents.

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**This equation can be written in another form:**

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Which has more power?

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**Work is transferred to gravitational potential Energy**

A 60kg man takes 5 seconds to run up a set of stairs at a constant velocity. What was the man’s power? Work is transferred to gravitational potential Energy Mass = 60kg 15m

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A man pushes a 12kg crate horizontally with a constant velocity of 2m/s. The coefficient of friction between the crate and the floor is What power does the man have as he pushes the crate? F f 120N -120N

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**A crane hoist lifts a load of 1000kg a vertical distance of 25m in 9**

A crane hoist lifts a load of 1000kg a vertical distance of 25m in 9.0s at a constant velocity. What is the useful power output of the crane?

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If the hoist motor of the previous crane is rated at 70hp, what percentage of this power goes to useful work?

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**Efficiency – A measure of what you get out for what you put in.**

The Efficiency of a typical car is about 25%!

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The motor of an electric drill with an efficiency of 80% has a power of input of 600 W. How much useful work is done by the drill in a time of 30 seconds?

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