AP Power and Eficiency

You should finish the Power and Efficiency worksheet tonight.

Here it is for those who lost it.
Power and Efficiency Worksheet

Power equals the rate at which work is done or the rate at which energy is transferred from one system or object to another. it is measured in Watts (W)

Power = Work/Dt.

Power x Dt = energy

Power provides an indication of how rapidly work can be done. If you must raise a large mass quickly, you need a more powerful motor to raise it then you would need if you needed to raise it slowly.

Efficiency is the ratio of useful work obtained to the work or energy put in to the effort. This is also described as work out /work in

eff = Woutput/Winput one symbol used for efficiency is , the Greek letter eta.

Examples: a) What is the power required to lift a 1kg mass 15 m in 5 sec?


b) The motor receives 45 W from the power company. What is the efficiency of the motor and the lifting machine? Since the useful power obtained is 30 W and the input power is 45 W, the efficiency is 30/45 = 0.67 or 67%.

1) What power is required to lift 20 kg 25 m in 30 sec?

2) How much power is required to change the kinetic energy of a car by 1,200,00J in 3 seconds? (ignore friction, etc.)

3) How much power is required to accelerate a 1400 kg car from 15 m/s to
20 m/s in 3 seconds?

4) If the engine actually delivers a power of 120,000 W during this time and the increase in kinetic energy is considered the only useful work, what is the efficiency of the process?

5) A light bulb receives 60 W of power for 20 minutes. How many J of energy did it receive?