1. Energy, Work & Power
Sec. 3.5, 3.7, 3.9

2. Is he doing any WORK?
He’s applying a FORCE, but is anything moving as a result of his applied force? FN FA
Balance forces, no change
in motion

3. Energy Doing work require ENERGY! Energy is the ability to do work
Work is a transfer of energy

4. Work Work = force x displacement W = f x d
In every case in which work is done,
work requires 2 things:
A force is applied to an object
The object moves a distance in the direction of the force W f d

5. Example
When a weightlifter lifts a barbell, he does work on the barbell
When he is holding the barbell over his head,
he does no work
(on the barbell)

6. Work or NO work A lifeguard sitting on her stand _____
Mowing the lawn. _____
Pushing on a wall _____
Carrying a box 10 meters. _____
Writing out your homework _____
Watching television _____
Eating a burger. _____
Running in place. _____

7. Units for Work The unit of work combines the unit of FORCE
Newtons
With the unit of DISPLACEMENT / DISTANCE
Meters
It’s called a Newton-meter
But…Since work requires ENERGY, then use the units for energy: Joule (J)
1 N of force to lift an m = ____ Joule of work

8. Work = F x d
How much WORK is done by applying 15 N of force to lift a chair 2 m above your head?

9. Example of Work
How much work is done to a 10 kg mass that is lifted 2 m?
Force weight = m x g
Work w = f x d

10. Who’s doing more work? They are both doing the SAME work:
Applying a FORCE on an object (dirt) to move it a distance.

11. They burn the same fuel…
Which has more POWER?
Lawnmower Engine
They burn the same fuel…
Jet Engine
30 minutes of work
…but at different rates
0.5 s of work

12. Power Power is the RATE at which WORK is done Power = work done time
or P = force x distance w p t

13. Hair dryer vs. Air drying your hair
What is the WORK that needs to be done?

14. Units for Power The unit for work is the Joule of energy
The unit for time is seconds
So...The unit for power is Joules/second (J/s)
Also called the Watt.
After James Watt

15. 1 J/s = 1 Watt
1000 W = kilowatt
1 million Watts = 1 mega- watt

16. practice A parent is pushing a child on a bicycle down the street.
If 125 J of work is done on a child’s bicycle for 120 seconds,
what was the power output of the parent? G U E S
Power

17. practice Anna walks upstairs on her way to class.
She weighs 500 N and the stairs go up 3.25 m.
What is her power output if she climbs the stairs in 12.6 seconds? G U E S
Work
Power

18. Hair dryer
1875 what?

19. more
powerful
dryer
hair

20. Power Which are you more tired…
Walking a box up a flight of stairs or running a box up a flight of stairs?
You are doing the same amount of work, so why are you more tired?
POWER
Power is the measure of how fast work is done

22. Work and Power Lab Prediction: Purpose: How much work is done,
how much power is used
to climb a flight of stairs.
Prediction:

23. Work = f x d Power = work time Weight = m x g (N)
Distance = 1st floor to 2nd floor
(meters)

24. Work = f x d Power = work time Weight = m x g (N)
Distance = 1st floor to 2nd floor
(meters)

25. Conclusion Questions Who did more work: walker or runner?
Explain what they did:
Who used more power?
Why?

26. Graph #1 Work done

27. Power graph
Joules

28. Things to think about….
If you push an object twice as far while applying the same force, you do
Four times as much work
The same amount of work
Twice as much work
½ as much work
A job is done slowly, while an identical job is done quickly. Both jobs require the same amount of work, but different amounts of
Energy
Power
Work
None of these

29. Things to think about….
Which requires more work: Lifting a 10 kg sack a height of 2 m, or lifting a 5 kg sack a height of 4 m?
The 10 kg sack
The 5 kg sack
They require the same amount of work
Whereas impulse is the time that a force acts,
work involves the
Time and distance that a force acts
Distance that a force acts
Acceleration that a force produces

30. When James Watt invented his steam engine, he was asked how many horses the engine could replace. To find out, Watt built a rope and pulley device which he could measure the time it took a horse to lift various weights.
Watt defined horsepower as the power a horse can produce steadily for a period of time.
One horsepower is equal to 746 Watts.

31. Horsepower A horse exerting 1 horsepower can raise 330 pounds of coal
100 feet in a minute.

32. Just for the fun of it, figure out how many horsepower the runner in your group has…to do this is simple.
Divide the power in watts that you figured in your data chart by That’s how much horsepower you have!
HP = power in watts
746

33. Most hp cars? #1. 2009 Bugatti Veyron (1001 hp)
# Lamborghini Murcielago (632 hp)
# Mercedes-Benz SLR McLaren (617 hp)
# Ferrari 599 GTB Fiorano (612 hp)
2009 Ford Escort (110 hp

34. Now you play! Ramps Game! http://www.gamegecko.com/ramps.php
Parachute Game!

35. Poster Activity Choose: Poster, Comic strip, short story…
Must include all of the following:
Momentum
Impulse
Force, time
Energy (KE and PE)
Work
Power