Understanding Work and Power in Science | 14.1 Work and Power

1. Chapter 14Work, Power, and Machines 14.1 Work and Power

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

3. What Is Work? • For a force to do work on an object, the force must cause the object to move. • no movement=no work done. Any force not acting in the direction of motion does no work on an object.

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

5. Calculating Work Using the Work Formula A weight lifter raises a 1600-newton barbell to a height of 2.0 meters.

6. Calculating Work Using the Work Formula A weight lifter raises a 1600-newton barbell to a height of 2.0 meters. W=fd W=1600 x 2 W= 3200J

7. Work Practice • A crane uses an average force of 5200 N to lift a girder 25 m • The brakes on a bicycle apply 125 N of frictional force to the wheels as it travels 14.0 m

8. Work Practice • A crane uses an average force of 5200 N to lift a girder 25 m W=fd W=5200N x 25m W = 130,000J • The brakes on a bicycle apply 125 N of frictional force to the wheels as it travels 14.0 m W=fd W=125N x 14m W = 1750J

9. Work Practice • While rowing, John exerts a force of 165 N per stroke while pulling the oar 0.8 m. How much work is done in 30 strokes? • A 900-N mountain climber scales a 100-m cliff

10. Work Practice • While rowing, John exerts a force of 165 N per stroke while pulling the oar 0.8 m. How much work is done in 30 strokes? W=fd W=165N x 0.8m W=132J • A 900-N mountain climber scales a 100m cliff W=fd W=900N x 100m W=90,000J

11. Work Practice • A turtle slowly crawls along carrying a bird feather on its back. It passes an elephant standing still with five large lions on its back. Who is doing more work, the turtle or the elephant? Explain.

12. Work Practice • A turtle slowly crawls along carrying a bird feather on its back. It passes an elephant standing still with five large lions on its back. Who is doing more work, the turtle or the elephant? Explain. W=fd The turtle, because the elephant is not moving. W=f x 0, therefore The elephant’s work is 0

13. What Is Power? How are work and power related? Poweris the rate of doing work. • To increase power, • increase the amount of work done in a given time • you can do a given amount of work in less time.

14. Calculating Power

15. 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.

16. 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?

17. 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? W=fd W=72N x 1m W=72J P=w/t P = 72J/2s P = 36Watts

18. 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?

19. Calculating Power • 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? W=fd W=200N x 1.5m W = 300J P=W/t P = 300J / 1s P = 300w

20. 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?

21. 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? P = w/t P = (15N x 1m) / 2s P = 7.5w

22. 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.)

23. 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.) P=w/t P = 10N x 0.5m / 1s P = 5w

24. Power Practice • John does 3960 J of work on the oars in 60 s • A mechanic does 5350 J of work to lift a car 0.5 m in 50 s • Anna weighs 565 N and goes up 3.25 m vertically by stairs. What is power if her time is 12.6 s?

25. Power Practice • John does 3960 J of work on the oars in 60 s P = 3960J / 60s = 66w • A mechanic does 5350 J of work to lift a car 0.5 m in 50 s P = 5350J /50s = 107w • Anna weighs 565 N and goes up 3.25 m vertically by stairs. What is power if her time is 12.6 s? P = 565N x 3.25 / 12.6s = 145.73w

26. James Watt and Horsepower Another common unit of power is the horsepower. One horsepower (hp) is equal to about 746 watts.

27. Power Practice Suppose you ride in a sleigh being pulled by horses at 16 km/h. Another sleigh being pulled at 10 km/h travels the same distance you do. Which horses are more powerful? How is speed related to power?

28. Power Practice Suppose you ride in a sleigh being pulled by horses at 16 km/h. Another sleigh being pulled at 10 km/h travels the same distance you do. Which horses are more powerful? How is speed related to power? 16km/h because they are capable of doing more work (traveling farther in less time)

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

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

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

37. Chapter 14Work, Power, and Machines 14.2 Work and Machines

38. Machines Do Work A machineis a device that changes a force. • Machines make work easier to do by • changing the size, direction, or distance over which a force acts.

39. Work Input and Work Output How are work input and work output related for a machine? Because of friction, the work done by a machine is always less than the work done on the machine.

40. Work Input and Work Output Work Input to a Machine • The force exerted on a machine is the input force. • The distance the input force acts through is the input distance. work input = input force X input distance

41. Work Input and Work Output Work Output of a Machine • force exerted by a machine = output force • The distance the output force is exerted through = output distance. work outputof a machine = output force X output distance.

42. Work Input and Work Output All machines use some amount of input work to overcome friction.

43. Assessment Questions • What is the output distance of a machine that requires 2 newtons of force exerted over 6 meters and whose output force is 4 newtons? • 2 meters • 3 meters • 6 meters • 12 meters

44. Assessment Questions • What is the output distance of a machine that requires 2 newtons of force exerted over 6 meters and whose output force is 4 newtons? • 2 meters • 3 meters • 6 meters • 12 meters

45. Assessment Questions 2. The work output of a machine is always greater than the work input to the machine. TrueFalse

46. Assessment Questions 2. The work output of a machine is always greater than the work input to the machine. TrueFalse

47. Chapter 14Work, Power, and Machines 14.3 Mechanical Advantage and Efficiency

48. Mechanical Advantage • The mechanical advantageof a machine is the number of times that the machine increases an input force. • MA greater than one multiplies the input force • MA less than one increases the distance and speed

49. Mechanical Advantage

50. Mechanical Advantage Ideal Mechanical Advantage The ideal mechanical advantage (IMA) of a machine is the mechanical advantage in the absence of friction. Because friction reduces mechanical advantage, engineers often design machines that use low-friction materials and lubricants.