1. Today’s special Vocab 14 check Test 13 results
Chapter 14 notes on work
14 HW I due next class
Prelab due next class
Movie

2. Work, Power, Machines
PAf.1: Explain how the law of conservation of energy applies to the transformation of various forms of energy (including mechanical energy, electrical energy, chemical energy, light energy, sound energy, and thermal energy).
PAa.1: Generate hypotheses on the basis of credible, accurate, and relevant sources of scientific information.
PAa.2: Use appropriate laboratory apparatuses, technology, and techniques safely and accurately when conducting a scientific investigation.
PAa.9: Use appropriate safety procedures when conducting investigations.

3. What Is Work? Force applied in a direction of movement
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.

4. Calculating Work

5. 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.

6. Calculating Power

7. 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.

8. 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?

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

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

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

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

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

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

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

16. 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.

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

18. 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.

19. 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

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 ANS: C

21. 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

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 ANS: D

23. 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

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 ANS: A

25. 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.

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. ANS: C

27. 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.
The work input equals the input force multiplied by the input distance.

28. Work Input and Work Output
Work Output of a Machine
The force exerted by a machine is called the output force.
The distance the output force is exerted through is the output distance.
The work output of a machine is the output force multiplied by the output distance.

29. Work Input and Work Output
W=Fd
Work input =
(input Force)(input distance)=(output Force)(output distance)
=Work Output

30. 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

31. 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 ANS: B

32. Assessment Questions
The work output of a machine is always greater than the work input to the machine. True False

33. Assessment Questions
The work output of a machine is always greater than the work input to the machine. True False
ANS: F, never

34. Today’s special HW I check; Q& A Lab: Pulleys Lab due next time
Lamb chop in space

35. Today’s special Turn in lab on front table. ISS tour 4 Leftovers
More notes on Machines
HW II due next
Movie (cont.)

36. A nutcracker is a machine that converts the input force applied to it into a larger force capable of cracking a nut.
Because it increases force, the nutcracker has a mechanical advantage greater than 1.

37. Mechanical Advantage
A loading ramp is a machine used to move heavy items into a truck.
The mechanical advantage of a ramp with a rough surface is less than that of a similar smooth ramp because a greater force is needed to overcome friction.

38. 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.

39. Calculating Mechanical Advantage

40. Calculating Mechanical Advantage
Calculating IMA
A woman drives her car up onto wheel ramps to perform some repairs. If she drives a distance of 1.8 meters along the ramp to raise the car 0.3 meter, what is the ideal mechanical advantage (IMA) of the wheel ramps?

41. Calculating Mechanical Advantage
Read and Understand
What information are you given?

42. Calculating Mechanical Advantage
Read and Understand
What information are you given?

43. Calculating Mechanical Advantage
Plan and Solve
What unknown are you trying to calculate?

44. Calculating Mechanical Advantage
Plan and Solve
What unknown are you trying to calculate?

45. Calculating Mechanical Advantage
Plan and Solve
What formula contains the given quantities and the unknown?
Replace each variable with its known value and solve.

46. Calculating Mechanical Advantage
Plan and Solve
What formula contains the given quantities and the unknown?
Replace each variable with its known value and solve.

47. Calculating Mechanical Advantage
Look Back and Check
Is your answer reasonable?

48. Calculating Mechanical Advantage
Look Back and Check
Is your answer reasonable?
The IMA must be greater than 1 because the input distance is greater than the output distance. The calculated IMA of 6 seems reasonable.

49. Calculating Mechanical Advantage
1. A student working in a grocery store after school pushes several grocery carts together along a ramp. The ramp is 3 meters long and rises 0.5 meter. What is the ideal mechanical advantage of the ramp?

50. Calculating Mechanical Advantage
1. A student working in a grocery store after school pushes several grocery carts together along a ramp. The ramp is 3 meters long and rises 0.5 meter. What is the ideal mechanical advantage of the ramp? Answer: IMA = Input distance/Output distance
IMA = 3 m/0.5 m = 6

51. Calculating Mechanical Advantage
2. A construction worker moves a crowbar through a distance of 0.50 m to lift a load 0.05 m off of the ground. What is the IMA of the crowbar?

52. Calculating Mechanical Advantage
2. A construction worker moves a crowbar through a distance of 0.50 m to lift a load 0.05 m off of the ground. What is the IMA of the crowbar? Answer: IMA = Input distance/Output distance
IMA = 0.5 m/0.05 m = 10

53. Calculating Mechanical Advantage
3. The IMA of a simple machine is 2.5. If the output distance of the machine is 1.0 m, what is the input distance?

54. Calculating Mechanical Advantage
3. The IMA of a simple machine is 2.5. If the output distance of the machine is 1.0 m, what is the input distance? Answer: Input distance = (IMA)(Output distance)
Input distance = (2.5)(1.0 m) = 2.5 m

55. Efficiency
Efficiency is usually expressed as a percentage.
For example, if the efficiency of a machine is 75 percent, then you know that 75 percent of the work input becomes work output.

56. Efficiency
If a machine requires 10.0 J of work input to operate, then the work output is 75% of
10.0 J.

57. Efficiency
Reducing friction increases the efficiency of a machine.
Roller bearings reduce the friction of the rotating wheels because rolling friction is less than sliding friction.
To further reduce the rolling friction, the roller bearings are also lubricated with grease.

58. Efficiency
Engineers analyze the flow pattern of a smoke trail to determine the fluid friction forces (air resistance) acting on the vehicle. Engineers use these data to optimize a vehicle's shape for maximum fuel efficiency.

59. Assessment Questions
Which statement about the actual mechanical advantage of a machine is true?
The actual mechanical advantage is greater than one if the input force is greater than the output force.
The actual mechanical advantage of a machine is greater than its ideal mechanical advantage when the output force is greater than the input force.
The actual mechanical advantage of a machine is always less than its ideal mechanical advantage.
The actual mechanical advantage of a machine is never affected by friction.

60. Assessment Questions
Which statement about the actual mechanical advantage of a machine is true?
The actual mechanical advantage is greater than one if the input force is greater than the output force.
The actual mechanical advantage of a machine is greater than its ideal mechanical advantage when the output force is greater than the input force.
The actual mechanical advantage of a machine is always less than its ideal mechanical advantage.
The actual mechanical advantage of a machine is never affected by friction. ANS: C

61. Assessment Questions
If a lever raises a large rock 0.1 meters when the other end of the lever moves downward 2 meters, what is the ideal mechanical advantage of the lever?
0.05
0.5 2 20

62. Assessment Questions
If a lever raises a large rock 0.1 meters when the other end of the lever moves downward 2 meters, what is the ideal mechanical advantage of the lever?
0.05
0.5 2
20 ANS: A

63. Assessment Questions
A machine is used to accomplish 300 J of work. If the efficiency of the machine is 60 percent, what is the necessary work input?
180 J
360 J
500 J
750 J

64. Assessment Questions
A machine is used to accomplish 300 J of work. If the efficiency of the machine is 60 percent, what is the necessary work input?
180 J
360 J
500 J
750 J ANS: C

65. Assessment Questions
The efficiency of any machine is less than 100% because of losses due to friction. True False

66. Assessment Questions
The efficiency of any machine is less than 100% because of losses due to friction. True False
ANS: T

67. The output of one device acts as the input of the next.

68. What are the six types of simple machines?
The six types of simple machines are the lever, the wheel and axle, the inclined plane, the wedge, the screw, and the pulley.

69. Levers
The screwdriver is being used as a first-class lever with a mechanical advantage greater than 1.
(Diagram is not drawn to scale.)

70. Levers
The wheelbarrow has its output force located between the input force and the fulcrum.
(Diagram is not drawn to scale.)

71. Levers
The output distance of the broom is greater than the input distance the hands move through.
(Diagram is not drawn to scale.)

72. Wheel and Axle
A wheel and axle is a type of simple machine consisting of two disks or cylinders with different radii.
Output
Output
Input
Input
Screwdriver shaft
Steering shaft
Screwdriver handle
Steering wheel

73. Inclined Planes
This long and winding road acts like an inclined plane.

74. Wedges and Screws
The wedge consists of two inclined planes that slope toward each other. The inclined planes force the wood fibers apart as the wedge is driven into the log.
Input force

75. Wedges and Screws
A screw is a simple machine made up of an inclined plane wrapped around a cylinder.

76. A fixed pulley changes only the direction of the input force.
Pulleys
Fixed Pulley
A fixed pulley changes only the direction of the input force.
4 N
4 N
4 N

77. Pulleys
Movable Pulley
Movable pulleys change both the direction and the size of the input force.
2 N
2 N
4 N

78. Pulley systems are made up of both fixed and movable pulleys.

79. Using Graphs What system requires the smallest input force to lift a 2500-N load?
Answer:

80. Using Graphs What system requires the smallest input force to lift a 2500-N load?
Answer: System C

81. Calculating Determine the actual mechanical advantage for each of the systems for a 2000-N input force.
Answer:

82. Calculating Determine the actual mechanical advantage for each of the systems for a 2000-N input force.
Answer: AMA = Output force/Input force
A: AMA = 1; B: AMA = 2; C: AMA = 8

83. Applying Concepts Which of the three systems shown in the graph consists of a single fixed pulley? Explain how you know.
Answer:

84. Applying Concepts Which of the three systems shown in the graph consists of a single fixed pulley? Explain how you know.
Answer: System A could be a fixed pulley because it has a mechanical advantage of 1.

85. Inferring Describe what happens to system B’s output force as the input force increases above 4000 N. How does this affect the mechanical advantage of the system at higher loads? Offer a possible cause for the performance shown in the graph.

86. Inferring Describe what happens to system B’s output force as the input force increases above 4000 N. How does this affect the mechanical advantage of the system at higher loads? Offer a possible cause for the performance shown in the graph.
Answer: The output force begins to decrease relative to the required input force. At higher loads the mechanical advantage is decreased. Increased friction at higher loads could be a cause.

87. Applying Concepts Using the mechanical advantage value from Question 2, determine the output force of system A for an input force of 8000 N.
Answer:

88. Applying Concepts Using the mechanical advantage value from Question 2, determine the output force of system A for an input force of 8000 N.
Answer: 8000 N

89. Compound Machines
This watch consists of a series of machines.
The output of one machine acts as the driving input for the next machine in the series.

90. A bar that is rotating about a fixed point is called a
Assessment Questions
A bar that is rotating about a fixed point is called a
fulcrum.
lever.
wedge.
compound machine.

91. A bar that is rotating about a fixed point is called a
Assessment Questions
A bar that is rotating about a fixed point is called a
fulcrum.
lever.
wedge.
compound machine. ANS: B

92. Assessment Questions
A 3-meter-long ramp is used to lift a piano to a moving truck, which is 1 meter off the ground. What is the ideal mechanical advantage of the ramp? 1 2 3 33

93. Assessment Questions
A 3-meter-long ramp is used to lift a piano to a moving truck, which is 1 meter off the ground. What is the ideal mechanical advantage of the ramp? 1 2 3 33
ANS: C

94. Assessment Questions
A machine, such as a bicycle, that combines many simple machines is known as a complex machine. True False

95. Assessment Questions
A machine, such as a bicycle, that combines many simple machines is known as a complex machine. True False
ANS: F, compound machine

96. Today’s special HW II check; Q & A Secret lives
Practice test: Work on mc in class, answer on Smartresponse!
Test 14 W P M next time!

97. Today’s special Turn in Practice Test on front desk
Test 14 W P M; Enter MC questions on Smartresponse; write 15 sentences & 2 essays on paper to turn in
Get vocab 15; work QUIETLY until everyone has had a chance to finish
Standards
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