Work and Power

Work Work is the transfer of energy that occurs when a force makes an object move. Work occurs when an object moves: the motion must be in the same direction as the applied force. Energy is always transferred from the object doing the work to the object that work is being done on. Work is done on an object ONLY when a force is being applied and the object moves.

Work Work is is equal to the force acting on the object times the distance moved. W=F * d The unit for work is Joules (J) and is defined as N*m or kilogram meters/s2

Does it take more work to run up a 3 flights of stairs or walk slowly? Which is more tiring?

Power Power is the amount of work done in a period of time, or the rate at which work is done. P=W/t The unit for power is Watts (W) W (Watts)≠W (work) Since work is the transfer of energy, power can also be defined as P=E/t where E is the energy.

Work and Power

Example Problems You push a refrigerator with a force of 100N. If you move the refrigerator 5 meters, how much work did you do? W=Fd F=100N d=5m W=(100N)(5m)=500N/m=500J

Example Problems A lawn mower is pushed with a force of 80N. If 12,000J of work are done in mowing the lawn, what is the total distance the mower was pushed? W=Fd d=W/F W=12,000J F=80N d=12,000J/80N=150m

Example Problems To lift a pile of books from a table requires 50J of work. How much power is needed to lift the books in .5s? P=W/t W=50J t=.5s P=50J/.5s=100W

Practice Problem It takes 100J of work to lift a box 10m. How much force is needed to lift the box? If the box is lifted in 20s, what is the power required? W=Fd F=W/d W=100J d=10m F=100J/10m=10N P=W/t=100J/20s=5W