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## Power

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**Objective: To define and apply the concepts of power and**mechanical efficiency. Power The rate at which work is done.**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.**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.**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? Mass = 60kg 15m**f**F 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 0.25. What power does the man have as he pushes the crate? 120N -120N**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?**If the hoist motor of the previous crane is rated at 70hp,**what percentage of this power goes to useful work?**Efficiency – A measure of what you get out for what you**put in. The Efficiency of a typical car is about 25%!**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?