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2. Standardized Test Prep
Resources
Chapter Presentation
Bellringers
Transparencies
Standardized Test Prep
Math Skills
Visual Concepts

3. Chapter 12 Table of Contents Section 1 Work, Power, and Machines
Work and Energy
Table of Contents
Section 1 Work, Power, and Machines
Section 2 Simple Machines
Section 3 What is Energy?
Section 4 Conservation of Energy

4. Chapter 12 Objectives Define work and power.
Section 1 Work, Power, and Machines
Chapter 12
Objectives
Define work and power.
Calculate the work done on an object and the rate at which work is done.
Use the concept of mechanical advantage to explain how machines make doing work easier.
Calculate the mechanical advantage of various machines.

5. Section 1 Work, Power, and Machines
Chapter 12
Bellringer
In your study of motion, you have learned that forces can cause motion. But in some cases, a force that is applied is balanced by another opposite force, and there is no net motion as a result. Look at the following illustrations, and identify the forces and motion in each one. (See illustrations on following slide.)
1. In one drawing, no motion is likely to occur. Which drawing is it?
2. Describe the forces that are acting in this diagram. If the person exerts slightly more force, what happens to the opposite force? Does it increase to match the new force of the person, stay the same, or decrease?

6. Section 1 Work, Power, and Machines
Chapter 12
Bellringer, continued

7. Section 1 Work, Power, and Machines
Chapter 12
What is Work?
Work is the transfer of energy to a body by the application of a force that causes the body to move in the direction of the force.
Work is done only when a force causes an object to move in the direction of the force. This is different from the everyday meaning of work.
Work Equation
work = force  distance
W = F  d

8. Chapter 12 Work Section 1 Work, Power, and Machines
Click below to watch the Visual Concept.
Visual Concept

9. Chapter 12 What is Work?, continued Work is measured in joules.
Section 1 Work, Power, and Machines
Chapter 12
What is Work?, continued
Work is measured in joules.
Because work is calculated as force times distance, it is measured in units of newtons times meters, N•m.
These units are also called joules (J). In terms of SI base units, a joule is equivalent to 1 kg•m2/s2.
Definition of joules
1 J = 1 N•m = 1 kg•m2/s2

10. Section 1 Work, Power, and Machines
Chapter 12
Math Skills
Work Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N?
1. List the given and unknown values.
Given: force, F = 190 N
distance, d = 2.0 m
Unknown: work, W = ? J

11. Chapter 12 Math Skills, continued 2. Write the equation for work.
Section 1 Work, Power, and Machines
Chapter 12
Math Skills, continued
2. Write the equation for work.
work = force  distance
W = f  d
3. Insert the known values into the equation, and solve.
W = 190 N  2.0 m = 380 N•m
W = 380 J

12. Section 1 Work, Power, and Machines
Chapter 12
Power
Power is a quantity that measures the rate at which work is done or energy is transformed.
Power Equation
Power is measured in watts.
A watt (W) is equal to a joule per second (1 J/s).

13. Chapter 12 Power Section 1 Work, Power, and Machines
Click below to watch the Visual Concept.
Visual Concept

14. Section 1 Work, Power, and Machines
Chapter 12
Math Skills
Power It takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process?
1. List the given and unknown values.
Given: work, W = 100 kJ = 1  105 J
time, t = 20 s
The distance of 18 m will not be needed to
calculate power.
Unknown: power, P = ? W

15. Chapter 12 Math Skills, continued 2. Write the equation for power.
Section 1 Work, Power, and Machines
Chapter 12
Math Skills, continued
2. Write the equation for power.
3. Insert the known values into the equation, and solve.

16. Machines and Mechanical Advantage
Section 1 Work, Power, and Machines
Chapter 12
Machines and Mechanical Advantage
Machines multiply and redirect forces.
Machines help people by redistributing the work put into them.
They can change either the size or the direction of the input force.
Different forces can do the same amount of work.
A machine allows the same amount of work to be done by either decreasing the distance while increasing the force or by decreasing the force while increasing the distance.

17. Section 1 Work, Power, and Machines
Chapter 12
Force and Work

18. Machines and Mechanical Advantage, continued
Section 1 Work, Power, and Machines
Chapter 12
Machines and Mechanical Advantage, continued
Mechanical advantage tells how much a machine multiplies force or increases distance.
Mechanical Advantage Equation

19. Section 1 Work, Power, and Machines
Chapter 12
Math Skills
Mechanical Advantage Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high.
1. List the given and unknown values.
Given: input distance = 5.0 m
output distance = 1.5 m
Unknown: mechanical advantage = ?

20. Chapter 12 Math Skills, continued
Section 1 Work, Power, and Machines
Chapter 12
Math Skills, continued
2. Write the equation for mechanical advantage.
Because the information we are given involves only distance, we only need part of the full equation:
3. Insert the known values into the equation, and solve.

21. Chapter 12
Section 2 Simple Machines
Objectives
Name and describe the six types of simple machines.
Discuss the mechanical advantage of different types of simple machines.
Recognize simple machines within compound machines.

22. Chapter 12
Section 2 Simple Machines
Bellringer
You may not think of a door as a simple machine, but it is one. It functions like a lever. Like other levers, when you exert a force on it (an input force), a force is exerted along the entire door (the output force).

23. Chapter 12 Bellringer, continued
Section 2 Simple Machines
Bellringer, continued
1. For all levers, one point along the lever stays still while the rest of the lever moves. This point is called the fulcrum. Where is the fulcrum of a door?
2. You can push at any point along the width of a door and it will open. Which position requires the least force: pushing the door near the hinges, in the middle, or near the side farthest from the hinges? (Hint: Which of these feels easiest to do?)
3. If you are trying to prop the door open, but your only doorstop is not very heavy, is it likely to work best near the hinges, in the middle, or near the side farthest from the hinges?

24. Chapter 12 The Lever Family
Section 2 Simple Machines
The Lever Family
The most basic machines are called simple machines.
The six types of simple machines are divided into two families.
The lever family:
The inclined plane family:
simple lever
simple inclined plane
pulley
wedge
wheel and axle
screw

25. The Lever Family, continued
Chapter 12
Section 2 Simple Machines
The Lever Family, continued
Levers have a rigid arm and a fulcrum.
Levers are divided into three classes.
All first-class levers have a fulcrum located between the points of application of the input and output forces.
In a second-class lever, the fulcrum is at one end of the arm and the input force is applied to the other end.
Third-class levers multiply distance rather than force. As a result, they have a mechanical advantage of less than 1.

26. Chapter 12
Section 2 Simple Machines
Levers

27. Chapter 12 Lever Section 2 Simple Machines
Click below to watch the Visual Concept.
Visual Concept

28. The Lever Family, continued
Chapter 12
Section 2 Simple Machines
The Lever Family, continued
Pulleys are modified levers.
The point in the middle of a pulley is like the fulcrum of a lever.
A single, fixed pulley has a mechanical advantage of 1.
Multiple pulleys are sometimes put together in a single unit called a block and tackle.
A wheel and axle is a lever or pulley connected to a shaft.
The steering wheel of a car, screwdrivers, and cranks are common wheel-and-axel machines.

29. Chapter 12
Section 2 Simple Machines
Pulleys

30. Chapter 12 Pulley Section 2 Simple Machines
Click below to watch the Visual Concept.
Visual Concept

31. The Inclined Plane Family
Chapter 12
Section 2 Simple Machines
The Inclined Plane Family
Inclined planes multiply and redirect force.
An inclined plane turns a small input force into a large output force by spreading the work out over a large distance.
A wedge is a modified inclined plane.
A screw is an inclined plane wrapped around a cylinder.

32. Chapter 12 Inclined Plane Section 2 Simple Machines
Click below to watch the Visual Concept.
Visual Concept

33. Chapter 12 Screws Section 2 Simple Machines
Click below to watch the Visual Concept.
Visual Concept

34. Chapter 12 Compound Machines
Section 2 Simple Machines
Compound Machines
A machine made of more than one simple machine is called a compound machine.
Examples of compound machines are:
scissors, which use two first class levers joined at a common fulcrum
a car jack, which uses a lever in combination with a large screw

35. Chapter 12
Section 3 What is Energy?
Objectives
Explain the relationship between energy and work.
Define potential energy and kinetic energy.
Calculate kinetic energy and gravitational potential energy.
Distinguish between mechanical and nonmechan-ical energy.

36. Chapter 12
Section 3 What is Energy?
Bellringer
In the chapter on matter, you learned that energy is conserved. Instead of being created or destroyed, it is just changed from one form to another. The energy of the sunlight that reaches Earth is the ultimate source of most of the energy around us. Look at the illustration below, and identify the types of energy involved.

37. Chapter 12 Bellringer, continued
Section 3 What is Energy?
Bellringer, continued
1. How did energy from sunlight provide the energy the girl needed to swing the bat? (Hint: What do you need to have energy?)
2. When the girl hits the ball, she exerts a force on it. Does she do work on the ball in the scientific sense of the term? Explain why.
3. After the girl hits the ball, the ball moves very fast and has energy. When the ball hits the fielder’s glove, it stops moving. Given that energy can never be destroyed but merely changes form, what happened to the energy the ball once had? (Hint: If you are the fielder, what do you hear and feel as you catch the ball?)

38. Chapter 12 Energy and Work Energy is the ability to do work.
Section 3 What is Energy?
Energy and Work
Energy is the ability to do work.
When you do work on an object, you transfer energy to that object.
Whenever work is done, energy is transformed or transferred to another system.
Energy is measured in joules.
Because energy is a measure of the ability to do work, energy and work are expressed in the same units.

39. Chapter 12 Potential Energy
Section 3 What is Energy?
Potential Energy
The energy that an object has because of the position, shape, or condition of the object is called potential energy.
Potential energy is stored energy.
Elastic potential energy is the energy stored in any type of stretched or compressed elastic material, such as a spring or a rubber band.
Gravitational potential energy is the energy stored in the gravitational field which exists between any two or more objects.

40. Chapter 12 Potential Energy Section 3 What is Energy?
Click below to watch the Visual Concept.
Visual Concept

41. Potential Energy, continued
Chapter 12
Section 3 What is Energy?
Potential Energy, continued
Gravitational potential energy depends on both mass and height.
Gravitational Potential Energy Equation
grav. PE = mass  free-fall acceleration  height
PE = mgh
The height can be relative.
The height used in the above equation is usually measured from the ground.
However, it can be a relative height between two points, such as between two branches in a tree.

42. Chapter 12
Section 3 What is Energy?
Math Skills
Gravitational Potential Energy A 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff?
1. List the given and unknown values.
Given: mass, m = 65 kg
height, h = 35 m
free-fall acceleration, g = 9.8 m/s2
Unknown: gravitational potential energy, PE = ? J

43. Chapter 12 Math Skills, continued
Section 3 What is Energy?
Math Skills, continued
2. Write the equation for gravitational potential energy.
PE = mgh
3. Insert the known values into the equation, and solve.
PE = (65 kg)(9.8 m/s2)(35 m)
PE = 2.2  104 kg•m2/s2
PE = 2.2  104 J

44. Chapter 12 Kinetic Energy
Section 3 What is Energy?
Kinetic Energy
The energy of a moving object due to the object’s motion is called kinetic energy.
Kinetic energy depends on mass and speed.
Kinetic Energy Equation
Kinetic energy depends on speed more than mass.

45. Chapter 12
Section 3 What is Energy?
Kinetic Energy Graph

46. Chapter 12 Kinetic Energy Section 3 What is Energy?
Click below to watch the Visual Concept.
Visual Concept

47. Chapter 12
Section 3 What is Energy?
Math Skills
Kinetic Energy What is the kinetic energy of a 44 kg cheetah running at 31 m/s?
1. List the given and unknown values.
Given: mass, m = 44 kg
speed, v = 31 m/s
Unknown: kinetic energy, KE = ? J

48. Chapter 12 Math Skills, continued
Section 3 What is Energy?
Math Skills, continued
2. Write the equation for kinetic energy.
3. Insert the known values into the equation, and solve.

49. Chapter 12 Other Forms of Energy
Section 3 What is Energy?
Other Forms of Energy
The amount of work an object can do because of the object’s kinetic and potential energies is called mechanical energy.
Mechanical energy is the sum of the potential energy and the kinetic energy in a system.
In addition to mechanical energy, most systems contain nonmechanical energy.
Nonmechanical energy does not usually affect systems on a large scale.

50. Other Forms of Energy, continued
Chapter 12
Section 3 What is Energy?
Other Forms of Energy, continued
Atoms and molecules have kinetic energy.
The kinetic energy of particles is related to heat and temperature.
Chemical reactions involve potential energy.
The amount of chemical energy associated with a substance depends in part on the relative positions of the atoms it contains.
Living things get energy from the sun.
Plants use photosynthesis to turn the energy in sunlight into chemical energy.

51. Other Forms of Energy, continued
Chapter 12
Section 3 What is Energy?
Other Forms of Energy, continued
The sun gets energy from nuclear reactions.
The sun is fueled by nuclear fusion reactions in its core.
Electricity is a form of energy.
Electrical energy is derived from the flow of charged particles, as in a bolt of lightning or in a wire.
Light can carry energy across empty space.
Light energy travels from the sun to Earth across empty space in the form of electromagnetic waves.

52. Chapter 12 Objectives Identify and describe transformations of energy.
Section 4 Conservation of Energy
Chapter 12
Objectives
Identify and describe transformations of energy.
Explain the law of conservation of energy.
Discuss where energy goes when it seems to disappear.
Analyze the efficiency of machines.

53. Section 4 Conservation of Energy
Chapter 12
Bellringer
You give yourself and your sled gravitational potential energy as you pull your sled to the top of a snowy hill. You get on board your sled and slide to the bottom of the hill, speeding up as you go.

54. Chapter 12 Bellringer, continued
Section 4 Conservation of Energy
Chapter 12
Bellringer, continued
1. When does the sled have the most potential energy? When does it have the least potential energy?
2. Where does the sled have the most kinetic energy? the least kinetic energy?
3. What happens to the relative amounts of potential and kinetic energy as the sled slides down the hill? What happens to the total energy?
4. After the sled reaches the bottom of the hill, it coasts across level ground and eventually stops. What happened to the energy the sled had?

55. Energy Transformations
Section 4 Conservation of Energy
Chapter 12
Energy Transformations
Energy readily changes from one form to another.
Potential energy can become kinetic energy.
As a car goes down a hill on a roller coaster, potential energy changes to kinetic energy.
Kinetic energy can become potential energy.
The kinetic energy a car has at the bottom of a hill can do work to carry the car up another hill.

56. Section 4 Conservation of Energy
Chapter 12
Energy Graphs

57. Energy Transformations, continued
Section 4 Conservation of Energy
Chapter 12
Energy Transformations, continued
Energy transformations explain the flight of a ball.
Mechanical energy can change to other forms of energy.
Mechanical energy can change to nonmechanical energy as a result of friction, air resistance, or other means.

58. Kinetic and Potential Energy Graph
Section 4 Conservation of Energy
Chapter 12
Kinetic and Potential Energy Graph

59. Energy Conversion in Automobile Engines
Section 4 Conservation of Energy
Chapter 12
Energy Conversion in Automobile Engines
Click below to watch the Visual Concept.
Visual Concept

60. The Law of Conservation of Energy
Section 4 Conservation of Energy
Chapter 12
The Law of Conservation of Energy
The law of conservation of energy states that energy cannot be created or destroyed.
Energy doesn’t appear out of nowhere.
Whenever the total energy in a system increases, it must be due to energy that enters the system from an external source.
Energy doesn’t disappear, but it can be changed to another form.

61. The Law of Conservation of Energy, continued
Section 4 Conservation of Energy
Chapter 12
The Law of Conservation of Energy, continued
Scientist study energy systems.
Boundaries define a system.
Systems may be open or closed.
When the flow of energy into and out of a system is small enough that it can be ignored, the system is called a closed system.
Most systems are open systems, which exchange energy with the space that surrounds them.

62. Conservation of Mechanical Energy
Section 4 Conservation of Energy
Chapter 12
Conservation of Mechanical Energy
Click below to watch the Visual Concept.
Visual Concept

63. Efficiency of Machines
Section 4 Conservation of Energy
Chapter 12
Efficiency of Machines
Not all of the work done by a machine is useful work.
A machine cannot do more work than the work required to operate the machine.
Because of friction, the work output of a machine is always somewhat less than the work input.
Efficiency is the ratio of useful work out to work in.
Efficiency is usually expressed as a percentage.
The efficiency of a machine is a measure of how much useful work it can do.

64. Efficiency of Machines, continued
Section 4 Conservation of Energy
Chapter 12
Efficiency of Machines, continued
Efficiency Equation
Perpetual motion machines are impossible.
Energy is always lost to friction or air resistance.
Machines need energy input.
Because energy always leaks out of a system, every machine needs at least a small amount of energy input to keep going.

65. Section 4 Conservation of Energy
Chapter 12
Math Skills
Efficiency A sailor uses a rope and an old, squeaky pulley to raise a sail that weighs 140 N. He finds that he must do 180 J of work on the rope in order to raise the sail by 1 m (doing 140 J of work on the sail). What is the efficiency of the pulley? Express your answer as a percentage.
1. List the given and unknown values.
Given: work input = 180 J
useful work output = 140 J
Unknown: efficiency = ? %

66. Chapter 12 Math Skills, continued
Section 4 Conservation of Energy
Chapter 12
Math Skills, continued
2. Write the equation for efficiency.
3. Insert the known values into the equation, and solve.

67. Mechanical Efficiency
Section 4 Conservation of Energy
Chapter 12
Mechanical Efficiency
Click below to watch the Visual Concept.
Visual Concept

68. Section 4 Conservation of Energy
Chapter 12
Concept Map

69. Understanding Concepts
Chapter 12
Standardized Test Prep
Understanding Concepts
1. What causes the force of acceleration of a Space Shuttle booster rocket?
A. exhaust gases pushing against the ground
B. exhaust gases pushing against the atmosphere
C. exhaust gases pushing against the rocket itself
D. exhaust gases pushing against other gas molecules in the rocket

70. Understanding Concepts, continued
Chapter 12
Standardized Test Prep
Understanding Concepts, continued
1. What causes the force of acceleration of a Space Shuttle booster rocket?
A. exhaust gases pushing against the ground
B. exhaust gases pushing against the atmosphere
C. exhaust gases pushing against the rocket itself
D. exhaust gases pushing against other gas molecules in the rocket

71. Understanding Concepts, continued
Chapter 12
Standardized Test Prep
Understanding Concepts, continued
2. Which of the following is true about an object in orbit?
F. It is beyond Earth’s gravity.
G. Its velocity does not change.
H. It is constantly falling toward Earth.
I. Its forward acceleration comes from air pressure.

72. Understanding Concepts, continued
Chapter 12
Standardized Test Prep
Understanding Concepts, continued
2. Which of the following is true about an object in orbit?
F. It is beyond Earth’s gravity.
G. Its velocity does not change.
H. It is constantly falling toward Earth.
I. Its forward acceleration comes from air pressure.

73. Understanding Concepts, continued
Chapter 12
Standardized Test Prep
Understanding Concepts, continued
3. Which of these statements describes the law of conservation of energy?
A. No machine is 100% efficient.
B. Energy is neither created nor destroyed.
C. The energy resources of Earth are limited.
D. The energy of a system is always decreasing.

74. Understanding Concepts, continued
Chapter 12
Standardized Test Prep
Understanding Concepts, continued
3. Which of these statements describes the law of conservation of energy?
A. No machine is 100% efficient.
B. Energy is neither created nor destroyed.
C. The energy resources of Earth are limited.
D. The energy of a system is always decreasing.

75. Understanding Concepts, continued
Chapter 12
Standardized Test Prep
Understanding Concepts, continued
4. A coal-burning power plant produces electrical energy with an efficiency of 30%. If the chemical energy produced by burning one kilogram of coal is 25,000,000 joules (J), how many joules of electrical energy are produced by the combustion of one gram of coal?

76. Understanding Concepts, continued
Chapter 12
Standardized Test Prep
Understanding Concepts, continued
4. A coal-burning power plant produces electrical energy with an efficiency of 30%. If the chemical energy produced by burning one kilogram of coal is 25,000,000 joules (J), how many joules of electrical energy are produced by the combustion of one gram of coal?
Answer: 7500 J

77. Chapter 12 Reading Skills
Standardized Test Prep
Reading Skills
When astronauts first landed on the moon, cameras sent back pictures of them jumping. They were able to jump much higher than on Earth even though they were wearing very heavy space suits. Muscles generate the same amount of force on Earth, on the moon, or in space.
5. Analyze why the astronauts could jump higher on the moon without exerting any additional force.

78. Reading Skills, continued
Chapter 12
Standardized Test Prep
Reading Skills, continued
[See reading passage on previous slide.]
5. Analyze why the astronauts could jump higher on the moon without exerting any additional force.
Answer: Although the force of gravity is less than on Earth, the astronauts’ muscles are able to exert the same force as on Earth.

79. Interpreting Graphics
Chapter 12
Standardized Test Prep
Interpreting Graphics
6. If the input force on this pulley system is 100 N, what is the output force?
F. 100 N
G. 200 N
H. 300 N
I. 400 N

80. Interpreting Graphics, continued
Chapter 12
Standardized Test Prep
Interpreting Graphics, continued
6. If the input force on this pulley system is 100 N, what is the output force?
F. 100 N
G. 200 N
H. 300 N
I. 400 N

81. Interpreting Graphics, continued
Chapter 12
Standardized Test Prep
Interpreting Graphics, continued
7. How could the amount of force required to raise the bucket be decreased even more?
A. Add additional pulleys.
B. Increase the length of the rope.
C. Thread the rope through the pulleys in opposite order.
D. Increase the amount of force on the free end of the rope.

82. Interpreting Graphics, continued
Chapter 12
Standardized Test Prep
Interpreting Graphics, continued
7. How could the amount of force required to raise the bucket be decreased even more?
A. Add additional pulleys.
B. Increase the length of the rope.
C. Thread the rope through the pulleys in opposite order.
D. Increase the amount of force on the free end of the rope.