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

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Presentation on theme: "Power The rate at which work is done. Objective: To define and apply the concepts of power and mechanical efficiency."— Presentation transcript:

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

2 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.

3 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.

4 This equation can be written in another form:

5 Which has more power?

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

7 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? -120N 120N F f

8 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?

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

10 Efficiency – A measure of what you get out for what you put in. The Efficiency of a typical car is about 25%!

11 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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