1. 14-1 cornell p. 412 What is work Calculating work What is power?
Calculating power?
James Watt and Horsepower.

2. The weight lifter applies a large force to hold the barbell over his head.
Because the barbell is motionless, no work is done on the barbell.

3. Work and Power
Work- the product of force and distance.
Work Requires motion
For a force to do work on an object, some of the force must act in the same direction as the object moves. If there is no movement, no work is done.
Any part of a force that does not act in the direction of motion does no work on an object.

4. What Is Work?
Work is done when a force acts on an object in the direction the object moves. Work is done when the weightlifter exerts an upward force to raise the barbell.

5. What Is Work?
Work Requires Motion
The weight lifter does no work on the barbell as he holds it over his head. The force applied to the barbell does not cause it to move.

6. Work Depends on Direction
If all of the force acts in the same direction as the motion, all of the forces do work.
If part of the applied force acts in the direction of motion, that part of the force does work.
If none of the force is applied in the direction of the motion, the force does no work.

7. All of the force does work on the suitcase.
What Is Work?
All of the force does work on the suitcase.
Force
Direction of motion
Force and motion in the same direction

8. All of the force does work on the suitcase.
What Is Work?
All of the force does work on the suitcase.
The horizontal part of the force does work.
This force does work
Force
This force does no work
Direction of motion
Direction of motion
Force and motion in the same direction
Part of force in direction of motion

9. All of the force does work on the suitcase.
What Is Work?
All of the force does work on the suitcase.
The horizontal part of the force does work.
The force does no work on the suitcase.
This force does work
Force
This force does no work
Force
Direction of motion
Direction of motion
Direction of motion
Force and motion in the same direction
Part of force in direction of motion
Lifting force not in direction of motion

10. Calculating Work
Formula:
force is in newtons (N)
distance is in meters (m)
The joule (J) is the SI unit of work.
A joule is equal to 1 newton-meter.

11. Using the Work Formula
A weight lifter raises a 1600-newton barbell to a height of 2.0 meters.
Work = Force × Distance
Work = 1600 N × 2.0 m
Work = 3200 N·m = 3200 J

12. Power is the rate of doing work.
Doing work at a faster rate requires more power.
To increase power you can:
increase the amount of work done in a given time,
you can do a given amount of work in less time.

13. What Is Power?
Work is required to move snow from one location to another. A person using a shovel and a person using a snow blower can both do the work needed to remove the snow.
The snow blower can do the job much faster because it has more power.

14. What Is Power?
Because the snow blower can remove more snow in less time, it requires more power than hand shoveling does.

15. Calculating Power
Formula
work is measured in joules (J)
time is measured in seconds (s).
The SI unit of power is the watt (W)
A watt is equal to one joule per second.

16. You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box? ?
72 N 1m
2 sec P= f= d= t=
fxd
(w)
P t

17. A tow truck exerts a force of 11,000N to pull a car out of a ditch
A tow truck exerts a force of 11,000N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? ?
11,000N 5m
25s P= f= d= t=
fxd
(w)
P t

18. James Watt and Horsepower
Another common unit of power is the horsepower.
One horsepower (hp) is equal to about 746 watts.
James Watt ( ) used horsepower to compare the power outputs of steam engines he had designed.
Horses were a logical choice for comparison as they were the most commonly used source of power in the 1700s.

19. James Watt and Horsepower
The horse-drawn plow and the gasoline-powered engine are both capable of doing work at a rate of four horsepower.

20. In which of the following cases is work being done on an object?
Assessment Questions
In which of the following cases is work being done on an object?
pushing against a locked door
suspending a heavy weight with a strong chain
pulling a trailer up a hill
carrying a box down a corridor

21. In which of the following cases is work being done on an object?
Assessment Questions
In which of the following cases is work being done on an object?
pushing against a locked door
suspending a heavy weight with a strong chain
pulling a trailer up a hill
carrying a box down a corridor ANS: C

22. Assessment Questions
A tractor exerts a force of 20,000 newtons to move a trailer 8 meters. How much work was done on the trailer?
2,500 J
4,000 J
20,000 J
160,000 J

23. Assessment Questions
A tractor exerts a force of 20,000 newtons to move a trailer 8 meters. How much work was done on the trailer?
2,500 J
4,000 J
20,000 J
160,000 J ANS: D

24. Assessment Questions
A car exerts a force of 500 newtons to pull a boat 100 meters in 10 seconds. How much power does the car use?
5000 W
6000 W
50 W
1000 W

25. Assessment Questions
A car exerts a force of 500 newtons to pull a boat 100 meters in 10 seconds. How much power does the car use?
5000 W
6000 W
50 W
1000 W ANS: A

26. One horsepower is a unit of power equal to
Assessment Questions
One horsepower is a unit of power equal to
0.746 W.
1.0 W.
746 W.
2,000 W.

27. One horsepower is a unit of power equal to
Assessment Questions
One horsepower is a unit of power equal to
0.746 W.
1.0 W.
746 W.
2,000 W. ANS: C