1. Unbalanced forces cause acceleration
Where have we been?
Unbalanced forces cause acceleration
Balanced forces = constant velocity
We have learned that forces are vectors too.
We can work with forces on angles.

2. What your forces do – and how fast.
Work and Power
What your forces do
– and how fast.

3. What is Work?
It’s Energy transferred by a force
only done when a force causes motion in the same direction as the force acting on it.
Motion
Force

4. If not moving, no work is done.
What is Work?
It’s Energy transferred by a force
If not moving, no work is done.
No Motion
Force

5. What is Work?
It’s Energy transferred by a force
If moving in a direction different from force, no work is done.
Force
Motion

6. Formula:
Angle
W = F x d (cosθ)
Work
Force
distance

7. So, when does the angle matter?
When 0º, W = Fd (cos 0º)
cos 0 = 1,
so, W = Fd
Makes no difference
when moving same direction as pushed

8. So, when does the angle matter?
When 90º, W = Fd (cos 90º)
cos 90 = 0,
so, W = 0
Work is zero
when moving 90º to force

9. W = F x d (cosθ) Formula: Work (joules) Force (N) distance (m) Angle
(if not 0º, or 90º)
W = F x d (cosθ)
Work (joules)
Force (N)
distance (m)

10. Work done on an object increases the total energy of the object.
Remember . . .
Work is Energy transferred by a force
Work done on an object increases the total energy of the object.
Work done by an object decreases the total energy of the object.

11. Gravity (objects falling) Friction (opposing motion)
Work is performed by . . .
Gravity (objects falling)
Friction (opposing motion)
Humans or machines (applying a push or pull)

12. Net Work is The sum of work done by all forces acting on an object
Use appropriate signs
Remember that work by 90º forces is 0J, so don’t waste time solving for it.

13. Quick quiz! Which of these are examples of work?
A book falls off a table and free falls to the ground.
A rocket accelerates through space.
A boy pushes on a large boulder, but the boulder is too heavy for him to move.

14. How about this one?
This waiter carries a tray of food across the room. Is the waiter doing work on the tray?

15. How many kilo-Joules is this?
Calculate work
A crane lifts a steel beam with a mass of 1,500 kg.
Calculate how much work is done against gravity if the beam is lifted 50 meters in the air.
1) You are asked for the work and the time it takes to do the work.
2) You are given mass, height, and the work done per second.
3) Use the formula for work done against gravity, W = mgh.
4) Solve:
W = (1,500 kg)(9.8 N/kg)(50 m) = 735,000 J
At a rate of 10,000 J/sec, it takes: 735,000 ÷ 10,000 = 73.5 seconds to lift the beam.
Joules
How many kilo-Joules is this?

16. Check your understanding!
500 J J J

17. One more…
Ben carries a 200-N suitcase up three flights of stairs (a height of 10.0 m) and then pushes it with a horizontal force of 50.0 N at a constant speed of 0.5 m/s for a horizontal distance of 35.0 meters. How much work does Ben do on his suitcase during this entire motion?
3750 J

18. Work Power = time Power W P t Power is the rate at which work is done.
Ch 8 Energy Notes
Power
Power is the rate at which work is done.
Work
Power =
time W P t

19. Work Power = time Power W P t Power is the rate at which work is done.
Ch 8 Energy Notes
Power
Power is the rate at which work is done.
Work
Power =
time W P t

20. Power Units of power: 1 Joule/sec = 1 Watt 1000 Watts = 1 kilowatt
Ch 8 Energy Notes
Power
Units of power: 1 Joule/sec = 1 Watt
1000 Watts = 1 kilowatt

21. Question 1
A woman performed 278 J of work in 40 seconds. Calculate her power.

22. Question 2
A pulley is used to lift a 620 N box of construction equipment 24 m straight up into the air. The pulley system accomplishes this in 18 seconds. Calculate the power of the pulley system

23. Question 3
Consider a box that is slid 12 m across the room. Two people apply a force of 19 N each. It takes them a total of 11 seconds to accomplish the feat. What is the combined power of the people?

24. Question 4
A certain generator performs kJ of work in 2.3 hours. How much power does the generator have? (Pay close attention to units!)

25. Question 5
A man pulled a sled at an angle of 32o. The force he exerted was 106 N until he had moved the sled 95 m. It took him 312 s to accomplish this feat. Calculate his power.