1. Measures of Effort & Motion; Conservation Laws
4/23/2008
Work, Energy, Power
Measures of Effort & Motion;
Conservation Laws
Lecture 9

2. What is Work? Pushing a car? Picking up a weight? Holding a weight up?
11/23/2017

3. What is Work? You do work on an object when you move it.
The rate at which you do work is your power output.
When you do work on an object, you transfer energy from one object to another.

4. Work Done by a Constant Force
The definition of work, when the force is parallel to the displacement:

5. Work work = force  displacement W = F · d
4/23/2008
Work
work = force  displacement
W = F · d
Work can be done by you, as well as on you
Work is a measure of expended energy
Lecture 9

6. Work = Fx x dx = F x cosθ x d A m/s^2 B kg * m^2 / s^2 C N * m/s^2
Units of Work
Work = Fx x dx = F x cosθ x d
What are the SI units for Work?
A m/s^2
B kg * m^2 / s^2
C N * m/s^2

7. Work = Fx x dx = F x cosθ x d A m/s^2 B kg * m^2 / s^2 C N * m/s^2
Units of Work
Work = Fx x dx = F x cosθ x d
What are the SI units for Work?
A m/s^2
B kg * m^2 / s^2
C N * m/s^2

8. Work is measured in Joules
Joules are the unit of energy
One joule of work is done when a force of 1 N acts on a system over a displacement of 1m

9. W = F•D What is the work if: The force on the box is 10 Newtons
The distance pushed is 5 meters

10. W = F•D What is the work if: The force on the box is 10 Newtons
The distance pushed is 5 meters
W = Fx x dx
W = 10 x 5 = 50 J

11. How Much Work is Done?
A friend’s car is stuck on the ice. You push down on the car to provide more friction for the tires (Ff = FN) allowing the car’s tires to propel it forward 5 meters onto less slippery ground. How much work did you do?
11/23/2017

12. Work Done?
A friend’s car is stuck on the ice. You push down on the car to provide more friction for the tires (Ff = FN) allowing the car’s tires to propel it forward 5 meters onto less slippery ground. How much work did you do?
W = F x cosθ x d
W = F (cos 90°) d
W = F (0) d
W = 0 J
11/23/2017

13. Negative, Positive, Zero Work

14. What is the work if:
The force on the box is 8 Newtons
The distance pushed is 2 meters
The angle is 30 degrees

15. Only the force in the direction of the displacement “counts”
Only a component of the force is doing work
W = Fx x dx = F x cosθ x d

16. Work = F x cos x d W = Fx x dx = F x cosθ x d
What is the work if: The force on the box is 8 Newtons; the distance pushed is 2 meters; The angle is 30 degrees
W = 8 x cos(30) x 2 = J

17.
11/23/2017

18. Machines can make work easy (ramps, levers)
4/23/2008
Work
Machines can make work easy (ramps, levers)
Apply less force over larger distance for same work
Lecture 9

19. Working at an advantage
4/23/2008
Working at an advantage
Work = Force  Distance Work = Force  Distance
Lecture 9

20. Ramps Larger Force Small Force Short Distance Long Distance M
4/23/2008
Ramps
Larger Force Small Force
Short Distance Long Distance M
Lecture 9

21. Whiteboard Team Challenge
Grab a whiteboard and get ready to play…
“You better work!”
11/23/2017

22. 1) To Work or Not to Work
Is it possible to do work on an object that remains at rest?
1) yes
2) no

23. 1) To Work or Not to Work
Is it possible to do work on an object that remains at rest?
1) yes
2) no
Work requires that a force acts over a distance. If an object does not move at all, there is no displacement, and therefore no work done.

24. 2) Friction and Work I
A box is being pulled across a rough floor at a constant speed. What can you say about the work done by friction?
1) friction does no work at all
2) friction does negative work
3) friction does positive work

25. 2) Friction and Work I
A box is being pulled across a rough floor at a constant speed. What can you say about the work done by friction?
1) friction does no work at all
2) friction does negative work
3) friction does positive work
Friction acts in the opposite direction to the displacement, so the work is negative. Or using the definition of work: W = F d cos q since  = 180o, then W < 0. f N mg
displacement
Pull

26. 3) Friction and Work II Can friction ever do positive work? 1) yes
2) no

27. 3) Friction and Work II Can friction ever do positive work? 1) yes
2) no
Consider the case of a box on the back of a pickup truck. If the box moves along with the truck, then it is actually the force of friction that is making the box move.

28. 4) Play Ball!
In a baseball game, the catcher stops a 90-mph pitch. What can you say about the work done by the catcher on the ball?
1) catcher has done positive work
2) catcher has done negative work
3) catcher has done zero work

29. 4) Play Ball!
In a baseball game, the catcher stops a 90-mph pitch. What can you say about the work done by the catcher on the ball?
1) catcher has done positive work
2) catcher has done negative work
3) catcher has done zero work
The force exerted by the catcher is opposite in direction to the displacement of the ball, so the work is negative. Or using the definition of work (W = F d cos q ), since  = 180o, then W < 0. Note that because the work done on the ball is negative, its speed decreases.
Follow-up: What about the work done by the ball on the catcher?

30. 5) Tension and Work
A ball tied to a string is being whirled around in a circle. What can you say about the work done by tension?
1) tension does no work at all
2) tension does negative work
3) tension does positive work

31. 5) Tension and Work
A ball tied to a string is being whirled around in a circle. What can you say about the work done by tension?
1) tension does no work at all
2) tension does negative work
3) tension does positive work
No work is done because the force acts in a perpendicular direction to the displacement. Or using the definition of work: W = F d cos q since  = 90o, then W = 0. v T
Follow-up: Is there a force in the direction of the velocity?