1. Work and Power

2. Objectives: Define work and power
Calculate work done on an object and the rate at which work is done
Use ‘work’ and ‘power’ as science terms

3. Work and Power notes: In science, ‘work’ has a specific meaning
Work is ONLY done when a force causes a change in the position or motion of an object in the direction of the applied force
i.e. making something move because of force

4. Work or no work?

5. Work or no work?
Lifting a barbell over your head?

6. Work or no work?
Holding a barbell over your head?

7. Work or no work?
Is there work being done ON the tray?

8. THINK! What’s the difference between doing work and not doing work?
How do you think you might calculate ‘work’? (Hint: what HAS to happen for work to be done?)

9. WORK EQUATION:
Work = Force x distance
W = F x d

10. Work is measured in joules (J).
1 J = 1 N•m = 1 kg•m2/s2
1 joule is roughly equal to the
energy required to lift a small
apple 1 meter
*N measures force/weight
*m measures mass...
To find weight in N, need to use w=mg
James Prescott Joule

11. THINK!
Remember the bus-pull challenge? At what point in this clip is work being done? Why?

13. Practice:
A crane uses an average force of 5200 N to lift a crate 25 m. How much work does the crane do on the crate?

14. An apple weighing 1N falls a distance of 1m
An apple weighing 1N falls a distance of 1m. How much work is done on the apple by the force of gravity?

15. Challenge!
A mechanic uses a hydraulic lift to raise a 1200 kg car 0.5 m off the ground. How much work does the lift do on the car?

16. THINK!
Does running up a flight of stairs require more work than walking? Why?
What is the difference then?

17. POWER POWER is how much work is done in a given time
POWER: the rate at which work is done or energy is converted.
power = work / time P = W / t
Measured in watts (W)
746 watts = 1 horsepower

18. Practice
While rowing across the lake during a race, John does 3960 J of work on the oars in 60.0 s. What is his power output in watts?

19. Using a jack, a mechanic does 5350 J of work to lift a car 0
Using a jack, a mechanic does 5350 J of work to lift a car m in 50.0 s. What is the mechanic’s power output?

20. On the bottom of your notes:
Using their scientific meanings, make a sentence using ‘work’ and a sentence using ‘power’.

21. Exit Ticket #1-4

22. 1. What is the equation to find work?
W=N x d
W=F x d
W=N/d
W=F/d

23. 2. The brakes on a bicycle apply 125 N of frictional force to the wheels as the bicycle travels 14.0 m. How much work have the brakes done on the bicycle? a. 8.9 J b N c J d N

24. 3. It takes 100 J or work to lift a box 18 m
3. It takes 100 J or work to lift a box 18 m. If this is done in 20 s, what is the power output?
5.55 W
5 W
90 W
3.6 W

25. 4. Anna walks up the stairs on her way to class
4. Anna walks up the stairs on her way to class. She weighs 565 N and the stairs go up 3.25 m vertically. calculate her power output if she climbs the stairs in 12.6 s.
146 W
44.8 W
173.8 W
7119 W