CHAPTER 7 LESSON 1 Energy and Energy Transformations

CHAPTER 7

LESSON 1

Energy and Energy

Transformations

Forms of Energy

Key Concepts

What do you think? Read the three statements below and decide

whether you agree or disagree with them. Place an A in the Before column

if you agree with the statement or a D if you disagree. After you¡¯ve read this

lesson, reread the statements to see if you have changed your mind.

Before

Statement

After

1. A fast-moving baseball has more kinetic energy

than a slow-moving baseball.

? What is energy?

? What are potential and

kinetic energy?

? How is energy related to

work?

? What are different forms

of energy?

2. A large truck and a small car moving at the same

speed have the same kinetic energy.

3TUDY #OACH

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

What is energy?

Think of the last time you saw a fireworks display. When

fireworks explode, you can see bursts of color in the night

sky. Fireworks release energy when they explode. Energy is

the ability to cause change. The energy in the fireworks causes

the changes that you see as flashes of light and that you

hear as loud booms.

Energy also causes other changes. Plants use energy from

the Sun to make food for growth and other processes. Energy

can cause changes in the motions or positions of objects.

When a hammer hits a nail, energy from the hammer moves

the nail. The explosion of fireworks, the growth of a flower,

and the motion of a hammer involve energy.

Sticky Notes As you read,

use sticky notes to mark

information that you do not

understand. Ask your teacher

to explain.

WORD ORIGIN

energy

from Greek energeia, means

¡°activity¡±

Key Concept Check

1. Define What is energy?

Kinetic Energy¡ªEnergy of Motion

Have you ever been to a bowling alley? When you rolled

the ball and it hit the pins, a change occurred¡ªthe pins fell

over. This change occurred because the ball had a form of

energy called kinetic (kuh NEH tik) energy. Kinetic energy is

energy due to motion. All moving objects have kinetic energy.

The kinetic energy of a moving object depends on two

factors: the object¡¯s speed and its mass.

Reading Essentials

Energy and Energy Transformations

107

Speed, Mass, and Kinetic Energy

Visual Check

Speed = 15 m/s

Mass = 8,000 kg

2. Interpret Which car in

the figure has more kinetic

energy? Why?

KE

KE

Speed = 15 m/s

Mass = 1,500 kg

KE

Speed = 25 m/s

Mass = 1,500 kg

Kinetic Energy and Speed

Make a two-pocket book.

Organize information about

the forms of energy on

quarter sheets of paper and

put them in the pockets.

Speed is one factor that affects kinetic energy. The faster

an object moves, the more kinetic energy it has. The figure

above shows two cars and a truck moving along a highway.

All the vehicles have kinetic energy (KE) because they are

moving. However, each vehicle¡¯s speed helps determine the

amount of kinetic energy the vehicle has. The vertical bars

show the kinetic energy of each vehicle.

Kinetic Energy and Mass

ergy Potential Energ

Kinetic En

y

3. Define What is kinetic

energy?

Notice in the figure that the two cars have the same

mass. The car in front has more kinetic energy because it is

moving faster. The car in the back and the truck are moving

at the same speed. The truck has more kinetic energy than

the car in the back because the truck has more mass than

that car.

Potential Energy¡ªStored Energy

An object can have energy even when it is not moving.

If you hold a ball in your hand and then let it go, gravity

will cause the ball to fall to Earth. The gravitational interaction

between the ball and Earth causes a change to occur.

Before you dropped the ball, the ball had energy. This

form of energy is called potential (puh TEN chul) energy.

Potential energy is stored energy due to the interactions between

objects or particles. Potential energy has different forms:

gravitational potential energy, elastic potential energy, and

chemical potential energy.

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Energy and Energy Transformations

Reading Essentials

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Key Concept Check

The kinetic energy of a moving object also depends on its

mass. If two objects are moving at the same speed, the object

with more mass has more kinetic energy.

Gravitational Potential Energy

When you are holding a book, energy is stored between

the book and Earth. This type of energy is called

gravitational potential energy. If you lift the book higher,

the gravitational potential energy between the book and

Earth increases.

The gravitational potential energy stored between any

object and Earth depends on the object¡¯s mass and its height

above the ground. Dropping a bowling ball from a height of

1 m causes greater change than dropping a tennis ball from

the same height. When two objects are at the same height,

the one with more mass has more

Gravitational

gravitational potential energy.

Potential Energy

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

The two vases on the bookcase

are identical; however, they have

different potential energies because

they are at different positions

above the ground. The vase on the

top shelf of the bookcase has more

gravitational potential energy than

the vase on the bottom shelf. An

object that falls from a greater

height can cause a greater change

than an identical object that falls

from a lower height.

4. Relate Gravitational

potential energy depends on

which two factors? (Circle the

correct answer.)

a. speed and distance

b. mass and height above

the ground

c. mass and speed

Visual Check

Elastic Potential Energy

Another form of potential

energy is elastic (ih LAS tik)

potential energy. Elastic potential

energy is energy that is stored when

an object is compressed or stretched. When you jump on a

pogo stick, you compress the spring. This gives the spring

elastic potential energy. When the spring decompresses, it

pushes you into the air.

Stretching an object also stores elastic potential energy. When

you stretch a rubber band, elastic potential energy is stored in

the rubber band. When you release the rubber band, the stored

elastic potential energy changes into kinetic energy. The kinetic

energy causes the rubber band to snap back to its original shape.

Plucking the strings of a guitar, jumping on a trampoline,

and pulling back on the string of a bow give these objects

elastic potential energy by stretching. When the strings and the

trampoline return to their original positions, they cause change.

Reading Essentials

Reading Check

5. Identify The two vases

on the bookcase have the

same mass. Circle the vase

that has the greater

gravitational potential

energy.

6. Consider Which is not

an example of how an object

gains elastic potential energy

by stretching? (Circle the

correct answer.)

a. jumping on a pogo stick

b. pulling on a rubber band

c. jumping on a trampoline

Energy and Energy Transformations

109

Chemical Potential Energy

When you eat, you take in another form of potential

energy. Food and other substances are made of atoms joined

together by chemical bonds. Chemical potential energy is

energy stored in the bonds between atoms.

Key Concept Check

7. Compare In what way

are all forms of potential

energy the same?

Look at the figure below. The small balls in the figure

represent atoms that make up a glucose molecule. The lines

between the atoms represent chemical bonds. Chemical

potential energy is stored in these bonds. When you eat

food, chemical reactions within your body release chemical

potential energy stored in the food. Your body uses chemical

potential energy in foods for all its activities, such as

moving, thinking, and growing. Bonds between atoms in

other substances, such as gasoline, also store chemical

potential energy. People use the chemical potential energy

in gasoline to drive cars.

Glucose Molecule

Energy is stored in

the chemical bonds

between atoms.

Chemical

bond

Visual Check

Glucose

molecule

Energy and Work

A force is a push or a pull. When a force is applied to an

object, the object¡¯s kinetic and potential energy can change.

You can transfer energy by doing work. Work is the transfer of

energy that occurs when a force makes an object move in the direction

of the force while the force is acting on the object.

Look at the figure at the top of the next page. The girl

does work on the box as she lifts it. As she lifts the box onto

the shelf, the energy of the box changes. The work she does

transfers energy to the box. The energy of the box increases

because of the gravitational interaction between the box and

Earth. The box¡¯s potential energy increases as she lifts the

box higher. The vertical bars in the figure show the work

that the girl does (W) and the box¡¯s potential energy (PE).

Work depends on force and distance. You do work on an

object only if the object moves. Suppose the girl shown in

the figure tries to lift the box but cannot lift it off the floor.

She transfers no energy, so she does no work on the box.

110

Energy and Energy Transformations

Reading Essentials

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

8. Identify Highlight some

of the chemical bonds in the

figure.

Work

PE W

PE W

PE W

An object that has energy can also do work. What will

happen if the girl drops the box as she is moving it onto the

shelf? When the box hits the floor, it does work on the floor.

Some of the box¡¯s kinetic energy is transferred to the floor.

The girl will hear some of the energy as a loud crash and feel

some of the energy near her feet as the energy travels through

the floor. Because energy and work are connected, energy is

sometimes described as the ability to do work.

Visual Check

9. Determine When did

the transfer of energy take

place between the girl and

the box?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Other Forms of Energy

You have just learned about two forms of energy¡ªkinetic

energy and potential energy. Kinetic energy is energy due to

motion. Potential energy is stored energy. There are other

forms of energy as well. All forms of energy are measured in

units called joules (J). A softball dropped from a height of

about 0.5 m has about 1 J of kinetic energy just before it hits

the floor.

Key Concept Check

10. Analyze How is

energy related to work?

Mechanical Energy The sum of potential energy and kinetic energy

in a system of objects is mechanical energy. When you do work

on an object, you change the object¡¯s mechanical energy.

Think again about the girl moving the box shown above.

At what point did the mechanical energy of the box change?

The mechanical energy of the box increased when the girl

lifted it off the ground. Now think about a basketball game.

The mechanical energy of a basketball increases when a

player shoots the ball.

Sound Energy Musical instruments are just a few of the

many things that produce sound. When you pluck a guitar

string, the string vibrates and creates sound. You hear a

sound when sound waves produced by the vibrating guitar

string reach your ears. The energy that sound carries is sound

energy. Sound energy is produced by objects that vibrate.

Sound energy cannot travel through a vacuum such as the

space between Earth and the Sun.

Reading Essentials

11. Apply Imagine that

you push on a large rock. At

what point does your effort

change the rock¡¯s mechanical

energy?

Energy and Energy Transformations

111

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