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Pretest

1. How much work is done when a weightlifter holds a barbell motionless over his head?

2. Calculate the work done on a 2-N mass when it is lifted to a height of 2 m.

3. Calculate the average speed of a bicycle that travels 100 m in 20 s.

4. Is weight a force? What is the formula for calculating weight?

Chapter 15

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Pretest (continued)

5. How does the temperature of an object change when it is acted on by friction?

6. True or False: In a closed system, the loss of momentum of one object equals the gain in momentum of another object.

7. How is power related to work?

8. True or False: The amount of work done on a machine (work in) always equals the amount of work done by the machine (work out).

Chapter 15

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Interest Grabber

How Is Energy Related to Work?Energy is defined as the ability to do work. Recall that work is the product of force and distance. If a force acts through a greater distance, it has done more work. You can use work to measure changes in energy.

Place two identical books on a table so there is a gap of about 8 cm between the books. Place a sheet of notebook paper on the books so it covers the gap as shown. Now drop a penny from a height of 10 cm onto the paper above the gap. Note what happens. Next, drop the penny from a height of 30 cm and observe the results.

1. How did the height of the penny affect the distance the paper moved?

2. How did lifting the penny affect the work it did on the paper?

3. How did lifting the penny affect its energy?

Section 15.1

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Reading Strategy

Building Vocabulary

Section 15.1

a. kinetic energy

b. the energy of motion

c. gravitational potential energy

d. elastic potential energy

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Calculating Kinetic EnergySection 15.1

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Calculating Kinetic EnergySection 15.1

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Calculating Kinetic EnergySection 15.1

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Calculating Kinetic EnergySection 15.1

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Interest Grabber

How Can Energy Change Forms?Have you even seen a Rube Goldberg device? Goldberg was an award-winning cartoonist who drew complex series of devices that performed relatively simple acts. The devices were arranged so that the output of one device would act as the input of the next. Goldberg became so well known for his drawings that people all over the world hold contests to see who can build the most complicated device. Study the Rube Goldberg cartoon below and answer the questions.

1. List at least three kinds of energy in the device.2. Describe one change, in which energy from one form is

converted into energy of another form.

Section 15.2

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Reading Strategy

Relating Cause and Effect

Section 15.2

a. The gull drops the oyster, and the oyster’s gravitational potential energy is converted into kinetic energy as the oyster falls. (Air resistance can be ignored.)

b. The oyster strikes a rock and breaks, kinetic energy is converted into work (breaking the shell) and thermal energy. The kinetic energy and gravitational potential energy of the oyster are now zero.

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Conservation of Energy

Section 15.2

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Conservation of Energy

Section 15.2

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Conservation of Energy

Section 15.2

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Conservation of Energy

Section 15.2

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Interest Grabber

Using Wind and WaterWind and water are two useful sourcesof energy found in nature.

You can demonstrate how these energy sources are converted into kinetic energy by making a model of a windmill or a water wheel. Tape a pencil to an index card. Hold the pencil loosely at both ends. Blow onto one side of the index card and observe what happens. Next, hold the index card above a bucket. Have a classmate pour water from a pitcher onto one side of the index card.

1. What happened when you blew into the card? What happened when you held the card under running water?

2. How do you think people used the kinetic energy of a water wheel or a windmill to grind corn or other kinds of grain?

Section 15.3

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Reading Strategy

a. Nonrenewable energy resources include oil, natural gas and coal. They exist in limited quantities.

b. Renewable energy resources include hydroelectric, solar, geothermal, wind, biomass, and nuclear fusion.

c. Energy resources can be conserved by reducing energy needs and by increasing the efficiency of energy use.

Section 15.3

Identifying Main Ideas

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Wind Turbine Section 15.3

1. How much work is done when a weightlifter holds a barbell motionless over his head?

2. Calculate the work done on a 2-N mass when it is lifted to a height of 2 m.

3. Calculate the average speed of a bicycle that travels 100 m in 20 s.

4. Is weight a force? What is the formula for calculating weight?

4 J

Pretest Answers Chapter 15

Click the mouse button to display the answers.

No work is done.

5 m/s

Yes. W = mg.

Chapter 15Pretest Answers (continued)

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the temperature increases

Power is the rate at which work is done.

5. How does the temperature of an object change when it is acted on by friction?

6. True or False: In a closed system, the loss of momentum of one object equals the gain in momentum of another object.

7. How is power related to work?

8. True or False: The amount of work done on a machine (work in) always equals the amount of work done by the machine (work out).

Interest GrabberAnswers

1. How did the height of the penny affect the distance the paper moved?

The paper moved farther when the penny was dropped from a greater height.

2. How did lifting the penny affect the work it did on the paper?

Lifting the penny allowed it to do more work on the paper.

3. How did lifting the penny affect its energy?

Lifting the penny increased the penny’s energy.

Section 15.1

Interest GrabberAnswers

1. List at least three kinds of energy in the device.

There is kinetic energy, chemical energy, and gravitational potential energy.

2. Describe one change, in which energy from one form is converted into energy of another form.

The chemical energy of the cannon is converted into the kinetic energy of the peanut.

Section 15.2

Interest GrabberAnswers

1. What happened when you blew into the card? What happened when you held the card under running water?

The moving air and water caused the card and pencil to rotate.

2. How do you think people used the kinetic energy of a water wheel or a windmill to grind corn or other kinds of grain?

Answers will vary. Sample possible answer:The kinetic energy of the windmill or water wheel probably turned gears, which cause a millstone to turn.

Section 15.3

Chapter 15Go Online

Data Sharing

Self-grading assessment

Articles on energy and energy sources

For links on potential and kinetic energy, go to www.SciLinks.org and enter the Web Code as follows: ccn-2151.

For links on energy, go to www.SciLinks.org and enter the Web Code as follows: ccn-2152.