Presentation on theme: "The rate at which work is done."— Presentation transcript:
1 The rate at which work is done. Objective: To define and apply the concepts of power and mechanical efficiency.PowerThe rate at which work is done.
2 The power is the work done divided by the time it takes to do the work. Units:J/s = Watt1 horsepower =746 Watts1000 Watts = 1 kWKilowatt-hours (kwh) measure energy.
3 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.
6 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 EnergyMass = 60kg15m
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?Ff120N-120N
8 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?
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?