1. Power

2. Power How quickly work is done. Amount of work done per unit time.
If two people mow two lawns of equal size and one does the job in half the time, who did more work?
Same work. Different power exerted.
POWER = WORK / TIME

3. The watt
P = W/t
Joules/second
1 watt = 1 J/s

4. Vector or Scalar?
Power is a scalar quantity

5. watts Used to measure power of light bulbs and small appliances
An electric bill is measured in kW-hrs.
1 kilowatt = 1000 W

6. Horsepower (hp) = 745.5 watts
750 N

7. Derivation of alternate equation for power
P=W/t=Fd/t= Fv
d/t = v

8. Power Problems
A box that weighs 575 N is lifted up 20 m by a cable attached to a motor. This occurs in 10 s. What power is developed by the motor in W?
P=W/t=Fd/t=(575N)(20m)/10s = 1,150 W
2)You push a wheelbarrow a distance of 60 m at a constant speed for 25.0s by exerting a 145 N- force horizontally.
What power do you develop?
If you move the wheelbarrow twice as fast, how much power is developed?
a)P=W/t=Fd/t=(145N)(60m)/25s=348W
b)P=Fv so if the force is kept constant and the speed is doubled, the power will be doubled. Thus the power is 696 W

9. Power Problems
3. A 700 N Marine in basic training climbs a 10.0 m vertical rope at a constant speed in 8.00s. What is his power output? P=W/t=Fd/t=(700N)(10m)/8s= 875 W 4. A certain automobile engine delivers 22,400 W to its wheels when moving at a constant speed of 27.0 m/s. What is the resistive force acting on the automobile at this speed? P=Fv 22,400 W=F(27 m/s) F=829 N

10. THOUGHT QUESTION
An older model car accelerates from 0 to speed v in 10s. A newer, more powerful sports car accelerates from 0 to 2v in the same time. What is the ratio of the powers for the two cars if they have the same mass? The power of the new car is four times that of the old car