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Benchmarks—SC.A.2.3.3 Annually Assessed (pp. 539–541): The student knows that radiation, light,
and heat are forms of energy used to cook food, treat diseases, and provide energy; SC.B.1.3.1
Annually Assessed (pp. 535–540, 543): identifies forms of energy and explains that they can be
measured and compared; SC.B.1.3.2 (pp. 541, 543); SC.B.1.3.4 (p. 542); SC.B.2.3.1 (p. 542)
Also covers: SC.H.1.3.7 Annually Assessed (p. 543)
Energy Changes
Energy
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Explain what energy is.
Describe the forms energy takes.
Describe how energy is used.
Energy is a term you probably use every day. You might say
that eating a plate of spaghetti gives you energy, or that a gymnast has a lot of energy. Do you realize that a burning fire, a
bouncing ball, and a tank of gasoline also have energy?
What is energy? The word energy comes from the ancient
Energy causes all the changes that
take place around you.
Review Vocabulary
energy transfer: a change in
energy from one form to another
New Vocabulary
energy
kinetic energy
potential energy
thermal energy
calorie
law of conservation of
energy
FCAT Vocabulary
Figure 1 Lightning causes
dramatic change as it lights up
the sky.
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CHAPTER 18 Energy534
Kennan Ward/CORBIS
Greek word energos, which means “active.” You probably have
used the word energy in the same way. When you say you have a
lot of energy, what does this mean? Energy is the ability to cause
change. For example, energy can change the temperature of a pot
of water, or it can change the direction and speed of a baseball.
The energy in a thunderstorm, like the one shown in Figure 1,
produces lightning that lights up the sky and thunder that can
rattle windows. Energy can change the arrangement of atoms in
molecules and cause chemical reactions to occur. You use energy
when you change the speed of a bicycle by pedaling faster or
when you put on the brakes.
What does energy do?
Figure 2 Any moving object
has energy because it can cause
change.
Identify a change that the bowling
ball is causing.
Kinetic Energy
One soccer ball is sitting on the ground and another is
rolling toward the net. How does the energy of the moving ball
compare to the one at rest? A moving ball certainly has the ability to cause change. For example, a moving bowling ball shown
in Figure 2 causes the bowling pins to fall. A moving ball has
energy due to its motion. The energy an object has due to its
motion is called kinetic energy. A football thrown by a quarterback has kinetic energy. A sky diver or a leaf falling toward Earth
also has kinetic energy.
Mass, Speed, and Kinetic Energy Although moving
objects have kinetic energy, not all moving objects have the same
amount of kinetic energy. What determines the amount of
kinetic energy in a moving object? The amount of kinetic energy
an object has depends on the mass and speed of the object, as
shown in Figure 3 on the next page. Imagine a small rock and a
large boulder rolling down a hillside at the same speed. Which
would have more kinetic energy? Think about the damage the rock
and the boulder could do if they hit something at the bottom of the
hill. The large boulder could cause more damage, so it has more
kinetic energy. Even though the rock and the boulder were moving
at the same speed, the boulder had more kinetic energy than the
rock because it had more mass.
Kinetic energy also depends on speed. The faster a bowling
ball moves, the more pins it can knock down. When more pins
are knocked down, a greater change has occured. So the faster
the bowling ball moves, the more kinetic energy it has. Kinetic
energy increases as speed increases.
Food Contains Chemical
Energy You transform
energy every time you
eat and digest food. The
food you eat contains
chemical energy. Your
body changes this form
of energy into other
forms of energy that keep
your body warm and
move your muscles. The
amount of chemical energy contained in food is
measured in Calories.
Check some food labels
to see how many Calories
your food contains.
SECTION 1 Energy Changes
535
Alan Thornton/Getty Images
VISUALIZING KINETIC ENERGY
Figure 3
T
he amount of kinetic energy of a moving object depends
on the mass and the speed of the object.Energy is measured in units called joules (J).For example, the fastest measured speed a baseball has been thrown is about 45 m/s.The
kinetic energy of a baseball traveling at that speed is about 150 J.
▼ A 600-kg race car, traveling at
about 50 m/s, has about 5,000 times
the kinetic energy of the baseball.
▼ Earth’s atmosphere is continually bombarded by
▼
There is evidence that a meteorite 10 km in
diameter collided with Earth about 65 million years
ago and might have caused the extinction of
dinosaurs.The meteorite may have been moving
400 times faster than the baseball and would have
a tremendous amount of kinetic energy due to its
enormous mass and high speed—about
a trillion trillion joules.
particles called cosmic rays, which are mainly highspeed protons.The mass of a proton is about a 100
trillion trillion times smaller than the mass of the
baseball.Yet, some of these particles travel so fast,
they have nearly the same kinetic energy as the
baseball.
▼
A sprinter with a mass of
about 55 kg and running at 9 m/s
has kinetic energy about 15 times
greater than the baseball.
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CHAPTER 18 Energy
(tl)The Extinction of the Dinosaurs by Eleanor Kish, reproduced by permission of the Canadian Museum of Nature, Ottawa, Canada, (tr)W. Cody/CORBIS, (cr)William Swartz/Index Stock/PictureQuest, (bl)Duomo/CORBIS, (br)CORBIS
Potential Energy
Suppose the ski lift in Figure 4 takes a skier to the top of a
hill. The skier has no kinetic energy when she is standing at the
top of the hill. But as she skis down and moves faster, her kinetic
energy increases. Where does this kinetic energy come from?
Gravity pulls the skier down the hill. If the skier were standing
at the bottom of the hill, gravity would not start her moving, as
it does when she is at the top of the hill. When the skier’s position is at the top of the hill, she has a form of energy called
potential energy. Potential energy is energy that is stored
because of an object’s position. By using the ski lift to take her
to the top of the hill, the skier increased her potential energy by
changing her position.
Increasing Potential Energy When you raise an object
above its original position, it has the potential to fall. If it does
fall, it has kinetic energy. To raise an object, you have to transfer
energy to the object. The ski lift uses energy when it takes a skier
up a hill and transfers some of that energy to the skier. This
energy becomes stored as potential energy in the skier. As the
skier goes down the hill, the potential energy she had at the top
of the hill is converted to kinetic energy.
If the skier were lifted higher, her potential energy would
increase. The higher an object is lifted above Earth, the greater
its potential energy.
Why did the skier’s potential energy increase as
she was carried up the hill?
Figure 4 Potential and kinetic
energy change as the skier moves
up and down the slope.
Her potential energy
is largest at the top of
the hill.
The skier's potential energy
increases as the ski lift carries
her up the hill.
As she skis down the hill,
potential energy transforms
into kinetic energy.
Here her kinetic energy is
greatest and her potential
energy is smallest.
SECTION 1 Energy Changes
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SC.B.1.3.2
Comparing Potential
Energy
Procedure
1. Hold a tennis ball about
1.5 m above a hard surface.
2. Before the ball is dropped,
estimate the height the
ball will reach on each of
its first three bounces.
3. Drop the ball and have a
partner use a meterstick
to measure the height of
the ball’s first three
bounces.
Analysis
1. When did the ball have the
greatest and least amount
of potential energy?
Explain.
2. Infer why the height of the
ball differed in each bounce.
3. How did the mechanical
energy of the ball change
after each bounce?
Converting Potential and Kinetic Energy
When a skier skis down a hill, potential energy is transformed to kinetic energy. Kinetic energy also can be transformed into potential energy. Suppose you throw a ball straight
up into the air. The muscles in your body cause the ball to move
upward when it leaves your hand. Because it is moving, the ball
has kinetic energy. Look at Figure 5. As the ball gets higher and
higher, its potential energy is increasing. At the same time, the
ball is slowing down and its kinetic energy is decreasing.
What happens when the ball reaches its highest point? The
ball comes to a stop for an instant before it starts to fall downward again. At its highest point the ball has no kinetic energy
because it isn’t moving. All the kinetic energy the ball had when
it left your hand has been converted to potential energy. As a
result, the ball will go no higher. As the ball falls downward, its
potential energy is converted back into kinetic energy.
Mechanical Energy The sum of the kinetic and potential
energy of an object is the mechanical energy of the object. The
mechanical energy of the ball wouldn’t change if potential
energy is converted only into kinetic energy, and if kinetic
energy is converted only into potential energy. However, as the
ball moves through the air, the force due to air resistance acts on
the ball. This force converts a small amount of the ball’s
mechanical energy into another form of energy—thermal
energy. As a result, the mechanical energy of the ball when you
catch it is slightly less than when you threw it upward.
What is the mechanical energy of an object?
Figure 5 Energy is transformed
as a ball rises and falls.
KE PE
As the ball leaves the person’s hand, it is moving the
fastest and has maximum
kinetic energy.
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CHAPTER 18 Energy
KE PE
As the ball moves upward, it
slows down as its kinetic
energy is transformed into
potential energy.
KE PE
As the ball moves downward, it
speeds up as its potential energy
is transformed into kinetic
energy.
Forms of Energy
Besides kinetic energy and potential energy, there are other
forms of energy. Some of these energy forms are due to motion
and are a type of kinetic energy. Other energy forms are stored
energy and are a type of potential energy. Some of these forms
of can be transferred from one object to another.
Thermal Energy All matter contains particles, such as atoms
or molecules. The particles in matter are always moving. As a
result, these particles have energy that is due to their motion.
The energy of the particles in matter due to their continual
motion is thermal energy. The thermal energy in an object
increases when the object’s temperature increases.
How does the thermal energy of an object
change when its temperature increases?
A campfire is used to provide light
and warmth. A fire emits radiant
energy that you see as light and
feel as warmth.
Electrical Energy When you plug a hair dryer into an electrical outlet and turn it on, another form of energy is being used—
electrical energy. When the hair dryer is turned on, electrically
charged particles called electrons flow in various parts of the
hair dryer. The energy of these flowing electrons is electrical
energy. As electrons flow in a hair dryer, their electrical energy is
used to make an electric motor spin and to make parts of the
hair dryer hot.
Chemical Energy The food that you eat is a source of energy
that is used by your body. Food contains chemical compounds.
Chemical compounds are made of atoms that are bonded
together. The bonds between atoms store chemical energy.
Because chemical energy is energy that is stored, it is a type of
potential energy. Chemical energy is released when chemical
reactions occur. The chemical reactions that occur as your body
digests food release chemical energy. This energy is used by your
body to grow, to move, and in many other ways.
X rays are waves that transfer radiant energy as they move from
place to place. The radiant energy
transferred by X rays is used to
form images of the human body.
Radiant Energy When you sit in sunlight, you can feel the
warmth of the Sun on your skin. You feel warm because the Sun
gives off, or emits, a form of energy called radiant energy. The
radiant energy that comes from the Sun sometimes is called
solar energy. Radiant energy is energy that is transferred by
waves from one place to another. The waves emitted by the Sun
transfer radiant energy. These waves travel through space and
strike your skin, making it feel warm. Light also is a type of wave
that transfers radiant energy. Figure 6 shows examples of other
objects that give off radiant energy.
A microwave oven uses the radiant
energy transferred by microwaves
to cook food.
Figure 6 Campfires, X-ray
images, and microwave ovens all
use radiant energy.
Identify other objects that emit
radiant energy.
SECTION 1 Energy Changes
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(t)Getty Images, (c)Alfred Pasieka/Photo Researchers, (b)Stone/Getty Images
LA.B.2.3.4
Topic: Nuclear Reactions
in the Sun
Visit fl6.msscience.com for Web
links to information about the
nuclear reactions that occur inside
the Sun.
Activity Draw a diagram show-
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ing the structure of the Sun and
where in the Sun the nuclear reactions occur.
Annually Assessed
Benchmark Check
SC.B.1.3.1 Compare and contrast chemical energy and nuclear
energy.
Nuclear Energy The Sun emits an enormous amount of
radiant energy every second. This energy is produced by reactions that occur in the Sun. However, these reactions aren’t
chemical reactions. Instead, the Sun’s radiant energy is produced
by reactions that involve the nuclei of atoms. Nuclear energy is
energy stored in the nucleus of an atom. This energy is released
when changes occur in an atom’s nucleus.
Measuring Energy
Recall that in the SI system of units, time is measured in seconds and distance is measured in meters. In the SI system of
units, energy is measured using a unit called the joule (J). A student with a mass of 50 kg walking at a speed of 2 m/s has 100 J
of kinetic energy. The same student sitting on a ledge 1 m above
the ground has about 500 J of potential energy.
Sometimes it is more convenient to measure energy using
other units. Another unit for energy is the calorie. One calorie is
the amount of energy needed to warm one gram of water by 1°C.
One calorie (cal) is equal to about 4.19 J. A different energy unit
is used to measure the energy content of food. This unit is the
Calorie (with a capital C) and is equal to 1,000 cal, or 4,190 J.
A candy bar that has 200 Calories supplies your body with almost
838,000 J of energy.
Changing Forms of Energy
Figure 7 An electric fan converts
electrical energy into other forms
of energy.
Mechanical
energy
Changing Electrical Energy Many of the devices you use
Thermal
energy
Electrical
energy
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Digital Vision
When you throw a ball into the air, the kinetic energy of the
ball changes into potential energy as the ball moves upward. As
the ball falls, potential energy changes into kinetic energy as the
ball speeds up. As the ball rises and falls, one form of energy
changes into another form of energy.
Every day, all around you and inside you, energy is changing
from one form to another. Changes in forms of energy make a
car move and make headphones produce sound. Inside your
body chemical energy is changed to thermal energy, electrical
energy, and mechanical energy. All the events that you see
around you involve energy changing form.
CHAPTER 18 Energy
every day, like the one shown in Figure 7, change electrical
energy to other forms of energy. For example, hair dryers, CD
players, and refrigerators contain electric motors. In an electric
motor, electrical energy makes the shaft of the motor spin, so
that electrical energy is changed to mechanical energy. Hair dryers and electric stoves contain heating elements that change
electrical energy into thermal energy. A lightbulb produces light
by changing electrical energy to radiant energy.
Changing Chemical Energy A different
energy transfer is used by a stove that burns natural
gas. When a fuel like wood, gasoline, or natural gas
is burned, chemical energy is changed into thermal
energy and light, or radiant energy. Figure 8 shows
another example of chemical energy changing
form. When you light a candle, wax is burned to
produce thermal energy and light. In a car’s engine,
gasoline is burned to produce thermal energy. The
car converts this thermal energy to mechanical
energy that enables the car to move. Your body uses
food as fuel. Chemical reactions in your body
change the chemical energy stored in food to thermal energy and mechanical energy.
Using Energy
Where does energy come from? Even though energy changes
from one form to another when it is used, energy never is created
or destroyed. According to the law of conservation of energy,
energy never can be created or destroyed, but only can be
changed from one form to another.
Sometimes it might seem that energy is being lost. A book
sliding on a table slows down and stops. The book’s kinetic
energy decreased as it slowed down. Figure 9 shows what happens to this energy. The kinetic energy of the book is not
destroyed, but is changed into another form of energy—thermal
energy. Friction causes the kinetic energy of the book to be
changed into thermal energy. This thermal energy makes the
book, the table, and the surrounding air slightly warmer.
tacular display of chemical energy
changing to thermal energy and
radiant energy.
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The Law of Conservation of Energy
Figure 8 Fireworks are a spec-
FC
When you use a hair dryer, or toast bread in a
toaster, or listen to a CD, you are using energy. In
each case, you used electrical energy to do something—dry your hair, cook food, and make sound. When you
burn a candle or heat water on the stove you are using chemical
energy do something—provide light or heat water. Energy that
is used to do something or perform a task is useful energy.
Annually Assessed
Benchmark Check
SC.B.1.3.2 How much thermal
energy is produced in an electric
motor if 100 J of electrical energy
are converted into 92 J of mechanical energy?
Figure 9 As the book slides
along the table, kinetic energy is
changed into thermal energy.
When the book stops, all of its
kinetic energy has been changed
into thermal energy.
Thermal energy
SECTION 1 Energy Changes
541
Randy Faris/CORBIS
Thermal
energy
Batteries
Figure 10 The energy conver-
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sions in a flashlight produce some
thermal energy that is not useful
energy.
AT
Useful Energy Always Decreases
Light bulb
Annually Assessed
Benchmark Check
SC.B.2.3.1 Why does the
amount of useful energy decrease
when energy changes from one
form into another?
When energy is used, it usually is changed from
one form to another. For example, a battery in a flashlight converts chemical energy into electrical energy.
This electrical energy is changed into useful radiant
energy by the flashlight bulb. However, some of the
electrical energy also is changed to thermal energy, as
shown in Figure 10. When energy changes form,
some thermal energy is produced. Not all of this thermal energy
can be used to do something useful. As a result, when energy
changes form, the amount of useful energy is always less than the
initial amount of energy. Some energy always is changed into thermal energy that cannot be used to perform useful work.
When energy is changed from one form to another, it
becomes less concentrated and more spread out. For example,
the electrical energy in a flashlight is concentrated in the batteries. When the flashlight is used, the light and thermal energy
produced is transferred to the air surrounding the flashlight.
Compared to the energy in the batteries, the light and thermal
energy transferred to the air is spread out, and is less concentrated. All energy conversions that occur cause energy to
become more spread out and less concentrated.
Summary
Self Check
Energy
Energy is the ability to cause change.
Kinetic energy is the energy an object has
because of its motion. Potential energy is
stored energy.
Forms of Energy
Energy comes in different forms. Some of
these forms are thermal energy, electrical
energy, chemical energy, radiant energy, and
nuclear energy.
One form of energy can be changed into
another form of energy.
Using Energy
According to the law of conservation of
energy, energy cannot be created or
destroyed, but only can change form.
When energy changes form, some thermal
energy is produced and the amount of useful
energy decreases.
1. Describe the energy transformations that occur when a
lightbulb is turned on. SC.B.1.3.1
2. Explain how the total energy changes when a falling
rock hits the ground. SC.B.1.3.2
3. Infer on which part of a roller coaster a roller coaster
car has the greatest potential energy. SC.B.1.3.1
4. Determine which has the greater kinetic energy if both
are traveling at the same speed—a fully loaded truck
or a motorcycle. SC.B.1.3.1
5. Think Critically When a ball is thrown upward, how
does the height reached by the ball depend on its
initial speed? SC.B.1.3.2
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CHAPTER 18 Energy
6. Diagram the energy transformations that occur when
you eat breakfast, walk to the bus stop, and ride the
bus to school. SC.B.1.3.2
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