1. Physical Science Unit 3-5
Work, Power & Energy
Physical Science
Unit 3-5

2. Work Transfer of energy that occurs when a force makes an object move-
cover a distance
The object has to move!

3. Doing Work 2 Conditions A force must make an object move
Movement must be in the same direction as the force

4. Examples of Work

5. Examples of NO work

6. Work & Energy When work is done, energy is transferred
More on energy later…

7. W = Fd Calculating Work Work = force x distance moved
Work = Newton-meters
Work = Joules (J)

8. Example!
Ms. Z pushes a fridge with a force of 100 N. If she moves it 5 m, how much work did she do?
W = Fd
W = 100N x 5m
W = 500 J

9. Example!
How much work does a crane do if it lifts a 235-kg load of rocks up 2.5 meters?
*Hint: find weight first
Fw = m x g
Fw = 235 kg x 9.8 m/s/s
Fw = 2303 N
W = Fd
W = 2303 N x 2.5 m
W = J

10. Example!
If 1500 J is done on an object to move it 30 m, what amount of force was used to move the object?
W = Fd
F = W/d
F = 1500 J / 30 m
F = 50 N

11. Example!
How much work is done if a gusty wind is blowing on a stationary building with a force of 2000 N?
Is there movement?
No work is done!

12. Power! P = W/t The amount of work done in 1 second
The rate at which work is done
P = W/t
Power = work/time
Power = watts
1 watt = 1J/sec

13. Example Problem!
Reyna does 900 J of work pushing a sofa. It took her 5 seconds to move the sofa. What was her power?
P = W/t
P = 900 J/5 s
P = 180 Watts

14. Another Example!
Chuck climbs a hill in 60 seconds. His power was 175 watts. How much work did he perform?
P = W/t
W = Pt
W = 175 Watts x 60 s
W = 10,500 J

15. Power & Energy Energy is transferred during work
Power is the rate at which energy is transferred
P = E/t

16. Horsepower What is the work the horse does on the pulley?
3 m
1000 N
What is the work the horse does on the pulley?
If the horse does the work in 4 seconds, what is its power?
If 1 HP = 746 W, then what is its horsepower?
W= 1000 N x 3 m = 3000 J
P = 3000 J / 4 s = 750 W
750 W / 746 W = 1.01 HP

17. Work, Power & Horsepower

18. Machines
A device that makes doing work easier
Think simple!

19. Machines

20. How do machines work? Increase force
Increase distance over which a force can be applied
Change the direction of applied force

21. Increasing Force Example – a car jack
Force exerted by the jack is greater than the force you push down

22. Force & Distance Example – ramp
Using a ramp increases the distance over which work is done
Less force is needed

23. Changing Direction Example – wedge (ax) Exert a force downward
Resulting force splits the tree