1. Power is the rate of which work is done.
Power : is equal to the work done, divided by the time taken to do the work
P = W/t
Power is the rate of which work is done.

2. Power
Power = W/t (W = Fd) So
Power = Fd/t ( v = d/t)
Power = Fv
P =Fv

3. Units to measure Power Power is measured in Watts (W)
1 Watt is 1 J of energy transferred in 1 s. A watt of power is relatively small. Power is usually measured in kilowatts ( 1000 W)
In English system power is measured in horse power (HP)
1 HP = 746 W

4. Sample Problem 1 (P = W/t)
A box that weighs 575 N is lifted a distance of 20.0 m. If the job is done in 10 s, what is the power developed by the motor? (W = Fd)
W = 575 N x 20 m = J
P = J/10 s = 1150 watts

5. Sample Problem 2
You push a giant turkey a distance of 60 m at a constant speed for 25 s. If you exert 145 N of force:
What power do you develop?
If you moved the turkey twice as fast, how much power is developed?

6. Sample Problem 2
You push a giant turkey a distance of 60 m at a constant speed for 25 s. If you exert 145 N of force:
What power do you develop?
W = 145 N x 60 m = 8700 J
P = W/t = 8700J/25 s = 348 W

7. Sample Problem 2
You push a giant turkey a distance of 60 m at a constant speed for 25 s. If you exert 145 N of force:
If you moved the turkey twice as fast, how much power is developed?
Time would be 12.5 s, twice as fast, half the time
P = 8700 J / 12.5 s = 696 W ( twice as much power)

8. Sample Problem 3
An electric motor develops 65 kW of power as it lifts an elevator 17.5 m in 35 s. How much force does the motor exert?
P = W/t: x 10 3 Watts = W/35s
W = 2.28 x 106 J
W = Fd
2.28 x 10 6 J = F(17.5m)
F = 1.3 x 105 N

9. Sample Problem 4
A lawn roller is rolled across a lawn by a force of 115 N along the direction of the handle, which is 22.5o above the horizontal. If 64.6 watts of power is generated in 90 s, what distance is the roller pushed?
Fx = 115 cos 22.5
Fx = N
W = Fd
5814 J = (106.3N)d
d = 54.7 m
P = W/t
64.6 watts = W/90s
W = 5814 J
W = F d
115 N
22.50

10. Power Lab Power = W/t Power = Fv
Objective: To use your knowledge of work and power to measure the horsepower you generate when you walk up a set of stairs.
Power = W/t Power = Fv

11. Power Lab ( P = W/t) W = mgh P = W/t mg = your weight
weight in newtons = # lbs x 4.45 N/lb
h = vertical height of staircase
P = W/t
t= time to travel to top of stairs
meters h

12. Convert power from Watts to Horsepower (HP)
Power Lab
P = W/t ( in Watts or J/s)
Convert power from Watts to Horsepower (HP)
746 Watts = 1 HP
meters h

13. Power Lab ( P = Fv) F = mg v = h/t
weight in Newtons = # lbs x 4.45 N/lb
v = h/t
t= time to travel to top of stairs
meters h

14. Questions 3 and 4 Food calories burned climbing the stairs
1 Food calorie = 4186 J
How high would you have to climb to burn 100 food calories (100 food calories 1/3 of a candy bar)?

15. Power Lab Safety!!! Work in groups of three (one person time,
one record data, climber).
Be careful on stairs!
Make sure your shoes are tied.
Do not run, walk at a quick steady pace.
Wait until stairs are clear before going down.
Do not skip stairs.
No more than two climbers on the
stairs at one time
Wait quietly when your group is finished,
our class will return to the room together.
meters h

16. WARNING!!!
Quiz next class on Work and Power!!!