The weight lifter applies a large force to hold the barbell over his head.

1. 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.

2. When does a force do work? In science, workis the product of force and distance. What Is Work?

3. 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. What Is Work?

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. What Is Work? Work Depends on Direction • If all of the force acts in the same direction as the motion, all of the force does 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. What Is Work? • All of the force does work on the suitcase. Force Direction of motion Force and motion in the same direction

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

9. 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 This force does no work Force 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

11. Calculating Work Units of Work When using SI units in the work formula, the force is in newtons, and distance is in meters. The joule (J) is the SI unit of work. A joule is equal to 1 newton-meter.

12. Calculating Work 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

13. How are work and power related? 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, or you can do a given amount of work in less time. What Is Power?

14. 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.

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

16. Calculating Power

17. Calculating Power When using SI units in the power formula, work is measured in joules (J), and time is measured in seconds (s). The SI unit of power is the watt (W), which is equal to one joule per second.

18. Calculating Power Calculating Power 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?

19. Calculating Power Read and Understand What information are you given?

20. Calculating Power Read and Understand What information are you given?

21. Calculating Power Plan and Solve What formula contains the given quantities and the unknown?

22. Calculating Power Plan and Solve What formula contains the given quantities and the unknown?

23. Calculating Power Plan and Solve Replace each variable with its known value and solve.

24. Calculating Power Plan and Solve Replace each variable with its known value and solve.

25. Calculating Power Look Back and Check Is your answer reasonable?

26. Calculating Power Look Back and Check Is your answer reasonable? 36 watts is not a lot of power, which seems reasonable considering the box was lifted slowly, through a height of only 1 meter.

27. Calculating Power 1.Your family is moving to a new apartment. While lifting a box 1.5 m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this?

28. Calculating Power 1.Your family is moving to a new apartment. While lifting a box 1.5 m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this? Answer: Work = Force × Distance = 200 N × 1.5 m = 300 J Power = Work/Time = 300 J/1.0 s = 300 W

29. Calculating Power 2.You lift a book from the floor to a bookshelf 1.0 m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0 s?

30. Calculating Power 2.You lift a book from the floor to a bookshelf 1.0 m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0 s? Answer: Work = Force × Distance = 15 N × 1.0 m = 15 J Power = Work/Time = 15 J/2.0 s = 7.5 W

31. Calculating Power 3. You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used? (Hint: The suitcase moves 0.5 m/s. Consider how much work the force does each second and how work is related to power.)

32. Calculating Power 3. You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used? (Hint: The suitcase moves 0.5 m/s. Consider how much work the force does each second and how work is related to power.)Answer: Work = Force × Distance = 10.0 N × 0.5 m = 5 JPower = Work/Time = 5 J/1.0 s = 5 W

33. James Watt and Horsepower Another common unit of power is the horsepower. One horsepower (hp) is equal to about 746 watts. James Watt (1736-1819) was looking for a way 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.

34. 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.

35. 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

36. 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 corridorANS: C

37. 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

38. 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 JANS: D

39. 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

40. 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 WANS: A

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

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