Power. Rate at which work is done P = ΔW t Measured in Watts (W) Usually seen as kW – kilowatts Horsepower (746 Watts) P = Power (Watts) ΔW = work done.

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Power

Rate at which work is done P = ΔW t Measured in Watts (W) Usually seen as kW – kilowatts Horsepower (746 Watts) P = Power (Watts) ΔW = work done (J) t = time interval (s)

More Power… Also seen as…. P = F d Δt P = F Δ v Since W = Fd --- substitute F*d for W Since v = d substitute Δt

Real World Examples HorsepowerHousing Bill Lightbulbs

A 193 kg curtain needs to be raised 7.5m, at a constant speed, in as close to 5.0s as possible. The power ratings for three motors are listed as 1.0 kW, 3.5kW and 5.5kW. Which motor is best for the job?

1.A 1000 kg elevator carries a maximum load of kg. A constant frictional force of 4000 N retards the elevator’s motion upward. What minimum power, in kilowatts, must the motor deliver to lift the fully loaded elevator at a constant speed of 3.00 m/s 2.A car with a mass of 1500 kg starts from rest and accleerates to a speed of 18.0 m/s in 12.0 s. Assume that the force of resistance remains constant at 400N during this time. What is the average power developed by the car’s engine? 3.A rain cloud contains 2.66x107 kg of water vapor. How long would it take for a 2.00 kW pump to raise the same amount of water to the cloud’s altitude, 2.00 km? 4.How long does it take a 19 kW steam engine to do 6.8x107 J of work? 5.A 1500 kg car accelerates unifromly from rest to 10 m/s in 3s. a. What is the work done on the car in this time interval? b. What is the power delivered by the engine in this time interval?