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

2. What is Work?
Define Work:
The product of force and distance.

3. Work Requires Motion
What condition must exist in order 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.

4. Work Depends on Direction
The amount of work done on an object depends on what two things?
Direction of the force
Direction of the movement
Does all the force have to act in the direction of movement to do work? No
Think of an original idea where this is true.

5. Calculating Work How do you calculate work?
Multiply the force acting in the direction of motion by the distance the object moves
Work = Force x Distance
Tomorrow when I am gone, you will do math practice with this formula 

6. Units of Work What are the two ways you can label work? Newton * Meter
force (N) x distance (m)
Joule (J)
1 N*m = 1 J

7. Using the Work Formula
How much work does a 25 N force do to lift a potted plant from the floor to a shelf 1.5 m high?
Work = Force x Distance
Work = 25 N x 1.5 m
Work = 37.5 N*m
Work = 37.5 J

8. What is Power? Define Power: What are two ways to increase power?
The rate of doing work.
What are two ways to increase power?
Increase the amount of work done in a given time.
Do a given amount of work in less time.

9. How does doing work at a faster rate affect the power required?
More power is required.
Think of an original example of two things that do the same task, where one requires more power, but less time and the other requires less power, but more time.
Plowing a field with a horse & chisel
Plowing a field with a tractor

10. Calculating Power How do you calculate power?
Divide the amount of work done by the time needed to do the work.
Power = Work/Time
What are two ways you can label power?
Joules/seconds
work (J) / time (sec)
Watt (W)

11. You lift a large bag of flour from the floor to a 1 m high countertop, doing 100 J of work in 2 sec. How much power do you use to lift the bag of flour?
Power = Work/Time
Power = 100 J / 2 sec
Power = 50 J/sec
Power = 50 W

12. James Watt and Horsepower
What is another unit for power?
Horsepower (hp)
Compare this unit with watts.
1 horsepower = 746 watts

13. Section 14.1 Assessment
How much work is done when a vertical force acts on an object moving horizontally?

14. Section 14.1 Assessment
A desk exerts an upward force to support a computer resting on it. Does this force do work? Explain.

15. Section 14.1 Assessment
Two cars have the same weight, but one of the cars has an engine that is twice as powerful as the other. Which car can make it to the top of a mountain pass first?
Which car does more work to reach the pass?

16. Section 14.1 Assessment
You carry two heavy bags of groceries upstairs to your kitchen. Will you do more work on the bags if you carry them one at a time? Explain.

17. Section 14.1 Assessment
You lift a book to a bookshelf 1 m above the floor. How much power is used if the upward force is 15 N and you do the work in 2 seconds?

18. Chapter 14: Work, Power, and Machines
14.2 Work & Machines
Chapter 14: Work, Power, and Machines

19. Machines Do Work Define machine: What is the main purpose of machines?
A device that changes a force.
What is the main purpose of machines?
They make work easier to do.
In which ways can machines do this?
Change the size of the force needed.
Change the direction of the force.
Change the distance over which a force acts.

20. Increasing Force How can a machine increase a force?
Increase the distance over which a small force is exerted.
A small force exerted over a large distance becomes a large force exerted over a short distance.
Think of an original example of a machine that increases force:
Bolt Cutters vs. Scissors

22. Increasing Distance How can a machine increase distance?
Exert a greater force over a smaller distance.
A machine that decreases the distance through which you exert a force, increases the amount of force required.
Think of an original example of a machine that increases distance:
Gears on a bike
Broom? Golf club? Bat?

24. Changing Direction
How can a machine change the direction of the applied force?
Think of an original example of a machine that changes direction:
Levers
Pry bars

26. Work Input & Work Output
How does the work done ON a machine compare to the work done BY a machine?
Work done ON a machine is bigger than work done BY it
More work goes in than comes out.
What does friction have to do with this?
Friction causes work/force to be lost.
It can be lost as:
Heat
Light
Sound

27. Work Input to a Machine Define input force: Define input distance:
The force you exert on a machine.
Define input distance:
The distance over which you exert a force.
Define work input:
Work done by you.
Write an equation that uses the last three terms:
Work Input = Input Force x Input Distance

28. Work Output of a Machine
Define output force:
The force exerted by a machine
Define output distance:
The distance over which a machine exerts a force
Define work output :
The work done by a machine

29. Look at Figure 7 (pg. 419). What is the input distance?
The length of the path over which the oar handle moves
(the arc)
How does the input distance compare to the output distance?
It is less

30. Section 14.2 Assessment
A machine produces a larger force than you exert to operate the machine. How does the input distance of the machine compare to its output distance?

31. Section 14.2 Assessment
You do 200 J of work pulling the oars of a rowboat. What can you say about the amount of work the oars do to move the boat? Explain.

32. Section 14.2 Assessment
How can you increase the work output of a machine?

33. Section 14.2 Assessment
When you swing a baseball bat, how does the output distance (the end of the bat) compare with the input distance (the distance your hands move)? Why might it be useful to know this?

34. Section 14.2 Assessment
An ad claims that a new wrench reduces the force needed to tighten a bolt. If this ad is true, what do you know about the input distance?

35. 14.3 Mechanical Advantage & Efficiency
Chapter 14: Work, Power, and Machines

36. Mechanical Advantage Define Mechanical Advantage
The number of times the machine increases an input force

37. Actual Mechanical Advantage
Define AMA:
The ratio of the output force to the input force
Write the equation:
AMA = O. Force/ I. Force

38. Ideal Mechanical Advantage
Define IMA:
The MA of a machine in the absence of friction
Write the equation:
IMA = I. Distance/O. Distance

39. Why is the AMA always less than the IMA?
Because there is always friction
What can be done to make the AMA closer to the IMA?
Use low-friction materials
Ball bearings
Oil/grease
Stream-lining

40. Efficiency Define Efficiency: Write out the equation:
The percentage of the work input that becomes work output
Write out the equation:
Efficiency = Work O. / Work I. X 100%

41. Why is efficiency of any machine always less than 100%?
Because there’s always friction.
What are some ways to increase efficiency?
Ball bearings
Grease/Oil
Smoother surfaces…
Can you think up a machine that would have 100% efficiency?

42. Section 14.3 Assessment
You test a machine and find that it exerts a force of 5 N for every 1 N of force you exert operating the machine. What is the AMA of the machine?

43. Section 14.3 Assessment
How can 2 machines appear identical and yet not have the same AMA?

44. Section 14.3 Assessment
What information would you use to calculate the efficiency of a machine?

45. Section 14.3 Assessment
When is the IMA of a machine greater than 1?

46. Section 14.3 Assessment
Suppose you are an inventor in You are constructing a bicycle of your own design. What could you do to ensure your bicycle efficiently changes the work input into forward motion?

47. Section 14.3 Assessment
You have just designed a machine that uses 1000 J of work from a motor for every 800 J of useful work the machine supplies. What is the efficiency of your machine?

48. Section 14.3 Assessment
If a machine has an efficiency of 40% and you do 1000 J of work on the machine, what will be the work output of the machine?

49. Section 14.3 Assessment
A construction worker moves a crowbar through a distance of 0.5 m to lift a load 0.05 m off the ground. What is the IMA of the crowbar?

50. Section 14.3 Assessment
The IMA of a simple machine is 2.5. If the output distance of the machine is 1 m, what is the input distance?

51. Chapter 14: Work, Power, and Machines
14.4 Simple Machines
Chapter 14: Work, Power, and Machines

52. What are the 6 types of simple machines?
Wedge
Lever
Screw
Wheel & Axle
Pulley
Inclined Plane

53. Levers Define lever: Define fulcrum:
A rigid bar that is free to move around a fixed point
Define fulcrum:
The fixed point the bar rotates around

54. Levers are classified into 3 categories based on the locations of what?
The locations of the input force, output force, and fulcrum

55. Define input arm: Define output arm:
The distance between the input force and fulcrum
Define output arm:
The distance between the output force and the fulcrum

56. How do you calculate the IMA of a lever?
Divide the input arm by the output arm

57. First-Class Levers
Where is the fulcrum located on a first-class lever?
Between the input and output force
Draw and label a sketch of a first-class lever

58. What is the MA of a first-class lever?
Greater than, equal to, or less than 1
Name some examples of simple machines that use first-class levers:
Teeter-totter
Scissors

59. Second-Class Levers
Where is the output force located on a second-class lever?
Between the input force and fulcrum
Draw and label a sketch of a second-class lever:

60. What is the MA of a second-class lever?
Greater than 1
Requires less force, but more distance
Name some examples of simple machines
that use second-class levers:
Wheelbarrow
Nut cracker

61. Third-Class Levers
Where is the input force located on a third-class lever?
Between the fulcrum and the output force
Draw and label a sketch of a third-class lever:

62. What is the MA of a third-class lever?
Less than 1
The output distance is greater then the input distance
Name some examples of simple machines that use third-class levers:
Broom
Hockey stick
Baseball bat
Golf club

63. Wheel & Axle Define a wheel and axle:
A simple machine that consists of 2 disks or cylinders, each w/ a different radius
Draw and label a sketch of a wheel and axle:

64. How do you calculate the IMA of a wheel and axle?
Divide the radius (diameter) of the input force by that of the output force
What is the MA of a wheel and axle?
Greater or less than one
Depends which end is the input

65. Name some examples of simple machines that use a wheel and axle:
Steering wheel and column
Screwdriver

66. Inclined Planes Define an inclined plane:
Slanted surface along which a force moves an object to a different elevation
Draw and label a sketch of an inclined plane:

67. How do you calculate the IMA of an inclined plane?
The distance along the plane divided by the change in height
What is the MA of an inclined plane?
Greater than 1

68. Name some examples of simple machines that use inclined planes:
Moving ramp
Wheelchair ramp
Switchbacks on a mountain

69. Wedges Define a wedge: Draw and label a sketch of an wedge:
A V shaped object whose sides are 2 inclined planes sloped towards each other
Draw and label a sketch of an wedge:

70. What has a greater IMA: A thin wedge 8 cm long?
A thick wedge 8 cm long?

71. Name some examples of simple machines that use wedges:
Knife blade
Zipper
Axe blade

72. Screws Define a screw: Draw and label a sketch of a screw:
An inclined plane wrapped around a cylinder
Draw and label a sketch of a screw:

73. What has a greater IMA: A screw with close-together threads?
A screw with far-apart threads?

74. Name some examples of simple machines that use screws:
Nuts
Bolts
Corkscrew

75. Pulleys Define a pulley:
A simple machine that consists of a rope that fits into a groove in a wheel
How do you find the IMA of a pulley or a pulley system?
It is equal to the number of rope sections supporting the load being lifted

76. Fixed Pulleys Describe a fixed pulley:
Wheel attached at a fixed location
Rotates in place
Direction of force is changed, but the size of the force is not
Draw and label a sketch of a fixed pulley:

77. What is the MA of a fixed pulley?
Always 1
IF = OF (disregarding friction)
Name some examples of fixed pulleys:
Flag pole
Mini-blinds

78. Moveable Pulleys Describe a moveable pulley:
The pulley is attached to the object being moved
Draw and label a sketch of a moveable pulley:

79. What is the MA of a moveable pulley?
Greater than 1
Name some examples of moveable pulleys:
Sails on a ship
Skyscraper window washers’ platforms

80. Pulley Systems Describe a pulley system:
Fixed and moveable pulleys together
Draw and label a sketch of a pulley system:

81. What is the MA of a pulley system?
Greater than 1
Even larger than a moveable pulley
Name some examples of pulley system:
Train car lifters

82. Compound Machines Define a compound machine:
A combination of 2 or more simple machines that work together
Name some examples of compound machines:
Car engine
Washing machine
Watch
Scissors …