1. Work and Power

2. Goal of the class To relate work, force and displacement
To recognise the difference between “work” and work
Question of the day: If I hold a 10kg box above my head for 15secs how much work is done?

3. Work
Work has a distinct meaning in science that is often used incorrectly in normal speech (similar to weight)
Work is done on an object when a force causes a displacement
W = Fd w = work
F = force
d = displacement

4. Work
The SI unit of work is the Joule (J), named after James Prescott Joule
1Nm = 1J = 1kgm2/s2
1 J of work is performed by a force of 1N moving an object 1m

5. Work
Work is NOT done unless the force applied creates a change in the object’s location
Example a boy pulls a sled with a horizontal force of 100N for 10m, how much work is done on the sled?

6. Work
A man lifts a box with a force of 50N a height of 80cm. Calculate how much work he does on the box.
If he carries the box to a table that’s 5m away, how much work has he done now?
0.8m
Timer
W = 40J

7. Work

8. Quick review What is work? What are the units for work?
If I get really tired using a lot of force to push against a wall, am I doing work?
What is the equation for work?
What two things does work depend on?

9. Power How much time it takes to do a certain amount of work
Units of power are Watts (W)
Watt is the unit of power

10. Power
Both go up the same height
Watt is the unit of power

11. Power
Both the rock climber and the hiker are going to the same place, so they are doing the same amount of WORK.
However, it will probably take the rock climber a lot longer to get there, meaning that his POWER is much lower.

12. Power P = Power rating of the object (W)
W = Work done by the object (J)
t = Time it takes to do the work (s)
Watt is the unit of power joke
It will take more power to run up the stairs than to walk up them as the time is smaller.

13. Power
Example problem 1
A motor does 6000 J of work in 40 seconds. What is the motor’s power output?
Example problem 2
The amount of power required to move an object can be increased without changing the amount of work done. How?

14. Question of the day
Question of the day: If I hold a 10kg box above my head for 15secs how much work is done?
The answer is 0J as the box doesn’t move.

15. Vocabulary
Work is done on an object when a force causes a displacement. Units Joules (J)

16. Homework
Read Chapter 4 section 1 pages
Workbook pages 43-45