Download Motion and Forces

CHAPTER 8
LESSON 3
Motion and Forces
Forces
Key Concepts
What do you think? Read the two 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
5. To apply a force, one object must be touching
another object.
6. If an object is at rest, there are no forces acting
• What are different types
of forces?
• What factors affect the force
of gravity?
• What happens when forces
combine?
• How are balanced and
unbalanced forces related
to motion?
on it.
3TUDY#OACH
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
What is force?
Today you might have pushed open the classroom door
or pulled the zipper on your backpack. In both of these cases,
you are applying a force. A force is a push or a pull on an object.
Force has size and direction. Just as you used arrows to show
the size and direction of velocity and acceleration, arrows
can show the size and direction of a force. The unit for force
is the newton (N). You use about 1 N of force to lift a stick of
butter or a medium-sized apple.
Contact Forces
When a karate expert breaks a board, his or her hand
applies a force to the wood. A contact force is a push or a pull
one object applies to another object that is touching it. Contact forces
can be small, such as a finger pushing a button. They can also
be large, such as a wrecking ball crashing into a building.
Identify the Main Ideas Fold
a sheet of paper into three
columns. Label them (K ) for
what you already know about
forces, (W ) for what you want
to learn, and (L) for the facts
that you learned. Fill in the
third column after you have
read this lesson.
Key Concept Check
1. Apply What are some
examples of contact and
noncontact forces you have
experienced today?
Noncontact Forces
Have you ever held a magnet near a paper clip and made
the paper clip move without touching the two objects
together? A force that one object applies to another object without
touching it is a noncontact force. The force that pulls the paper
clip is a noncontact force. Another noncontact force, an
electrical force, sometimes causes socks to stick together
when they are pulled out of the dryer.
Reading Essentials
Motion and Forces
137
Gravity—A Noncontact Force
When you jump off a step, the force of Earth’s gravity pulls
you toward Earth. Did you know that your gravitational force
pulls Earth and other objects toward you? Gravity is an
attractive force that exists between all objects that have mass.
Mass is the amount of matter in an object. Both you and
Earth have mass, so both you and Earth pull on each other.
Why doesn’t Earth move toward you?
Key Concept Check
2. State What factors
Earth does move toward you. However, Earth’s mass is
much greater than your mass, so it is easier to see the effect
of Earth’s force on you than that of your forces on Earth.
The size of a gravitational force depends on the masses of
the objects and the distance between them.
affect the force of gravity?
Gravitational force depends on mass.
See the left side of the figure below. It shows that, if the
mass of an object increases, the gravitational force increases
between it and another object. The gravitational force
between you and Earth is large because of Earth’s large mass.
The force holds you on Earth’s surface. The gravitational
force between you and your pencil is small because you and
the pencil have relatively small masses.
Visual Check
3. Predict Look at the
objects on the left side of the
figure. What would happen to
the force of gravity if the mass
of the objects in A became as
large as the objects in B?
Gravitational force depends on distance.
Effect of Mass and Distance on Force of Gravity
Effect of Mass
on Force of Gravity
Effect of Distance
on Force of Gravity
C
A
1m
D
B
138
1m
1m
2m
The distances between the marbles in
diagrams A and B are the same. The
force of attraction between the marbles
in B is greater than in A because the
marbles in B have more mass.
The masses of the marbles in diagrams
C and D are the same. The force of
attraction between the marbles in D
is less than in C because the distance
between the marbles is greater.
Motion and Forces
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Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
See the right side of the figure below. It shows that the
gravitational force between two objects 1 m apart is four
times greater than the gravitational force between the same
objects when they are 2 m apart.
Mass and Weight in Different Locations
Location
Mass (kg)
Weight (N)
Earth
90
900
Moon
90
150
Interpreting Tables
4. Infer What would be the
weight of a girl on the Moon
if she weighs 600 N on Earth?
Mass and weight are different.
Weight is a measure of the gravitational force acting on an
object’s mass. Therefore, weight depends on the masses of
the objects and the distance between them. When comparing
the weight of two objects at the same location on Earth, the
object with more mass has a greater weight. The weights of the
same objects on the Moon are less because the mass of the
Moon is less. The relationship between weights and mass on
Earth and the Moon is shown in the table above. Because it
has less mass, the Moon’s gravity is only 1/6 that of Earth’s.
Therefore, an astronaut’s weight on the Moon is 1/6 the
astronaut’s weight on Earth.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Friction—A Contact Force
Rub your finger across your desk. Then rub it across a
piece of your clothing. What did you feel? It’s easy to run
your finger over your desk because it is smooth. On your
clothing, you felt a force called friction. Friction is a contact
force that resists the sliding motion of two surfaces that are touching.
Rough surfaces tend to produce more friction. Smooth
surfaces produce less friction.
5. Apply Which has more
friction between them: two
pieces of sandpaper or two
pieces of plastic?
Effects of Friction
Push your book across your desk. The book stops when
you stop pushing it. The force of friction acts in the opposite
direction of the book’s motion. A heavier book is more
affected by friction than a lighter one. If you want to move
a box of heavy books by sliding the box across the floor, you
have to use a large pulling force. The force of friction between
the box and the floor acts in the opposite direction to the
force of your pull. To move the box, your pulling force must
be greater than the friction force.
Air Resistance
Reading Check
6. Categorize Is air
resistance a contact force or
a noncontact force? Explain
your answer.
When you drop a sheet of paper, it slowly drifts downward.
Friction between the air and the paper’s surface slows its
motion. Air resistance is the frictional force between air and objects
moving through it. When you crumple the paper into a ball,
less surface area is in contact with the air. As air resistance
decreases, the ball of paper falls more quickly.
Reading Essentials
Motion and Forces
139
Combining Forces
7. Predict what would
happen if the dog’s force
increased to –60 N while the
girl’s force stayed at 50 N.
What would be the net force?
Key Concept Check
8. Describe What can
happen when forces
combine?
Suppose you need to pull your desk away from the wall
to get something that fell behind it. When you pull, the desk
will not move, so you ask a friend to help you. With both of
you pulling, the desk moves. You each applied a force to the
desk, and you applied it in the same direction. The combined
force was great enough to move the desk. When more than
one force acts on an object, the forces combine and act as
one force. The sum of all the forces acting on an object is
called the net force. When forces act in the same direction,
they add together to form one net force.
Sometimes forces act in opposite directions. Imagine
watching a dog pulling on its owner’s leash. The dog applies
a force to the left. The owner pulls the leash to the right,
applying an opposite force. When two forces act on the same
object in opposite directions, you must include the direction
of the forces when you add them. The positive direction is
usually to the right. The girl’s force on the dog’s leash is +50 N.
The dog’s force on the leash is the same size as the girl’s but
in the negative direction. The dog’s force is -50 N. The net
force on the leash is 50 N + (-50 N) = 0 N. The dog doesn’t
move.
Balanced Forces
9. Draw an arrow to show
the acceleration that would
result if the pull force on the
right side of the figure was
reduced to 150 N while the
friction force stayed the same.
What would be the net force?
Unbalanced Forces
When the net force on an object is not 0, the forces
acting on the object are unbalanced forces. The net force on
the right side of the figure below is 100 N to the right. The
forces acting on the object are unbalanced. Acceleration is in
the direction of the larger force.
Combining Forces
Balanced Forces
50 N
Unbalanced Forces
Acceleration
50 N
Friction = 100 N
The forces are balanced. They are equal
in size and opposite in direction.
50 N + (-50 N) = 0
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Motion and Forces
Pull = 200 N
The forces act in opposite directions,
but they are not balanced.
200 N + (-100 N) = 100 N
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Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Visual Check
If the net force on an object is 0 N, the forces acting on
the object are called balanced forces. Look at the figure below.
The net force on the left side of the figure is 0 N. The forces
acting on the object are balanced.
Unbalanced Forces and Acceleration
Reading Check
When you kick a soccer ball, its motion changes. The
forces on the ball are unbalanced. When unbalanced forces
act on an object, the object’s velocity changes. Unbalanced
forces can change either the speed or the direction of
motion.
10. Describe how
unbalanced forces affect
an object.
Change in Speed
Look at the figure below. In the top left image, the train is
pulling away from the station. The force of the engine pulling
the train forward is greater than the force of friction holding it
back.
Visual Check
The forces on the train are unbalanced, so it accelerates.
The train speeds up. The train accelerates in the direction of
the larger force, which is the forward pull of the engine.
11. Analyze What would
happen if the friction in the
bottom right image was
suddenly greater than the
force of the engine?
Change in Direction
When the train goes around a curve, as shown in the
bottom left image of the figure, the track exerts a sideways
force on the train’s wheels. These unbalanced forces change
the train’s motion by changing its direction and its velocity.
The train accelerates.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
The Effect of Balanced and Unbalanced Forces
Unbalanced Forces = Acceleration
Balanced Forces = Constant Motion
Changing speed
Resting
Gravity
Friction
from rails
Force
of engine
Force of track
Changing direction
Constant velocity
Force
of engine
Force from
track on wheels
Reading Essentials
Friction
from rails
Force
of engine
Motion and Forces
141
Balanced Forces and Constant Motion
Make a small four-door
shutterfold to record what
you learn about forces.
Contact
Forces
NonContact
Forces
Balanced
Forces
Unbalanced
Forces
Key Concept Check
12. Contrast How do
balanced and unbalanced
forces affect motion?
How do balanced forces affect an object’s motion? The
forces acting on a train that is sitting still on a track are
balanced. The force of gravity pulls the train down toward
Earth. The track pushes in an upward direction with an
equal force. If the forces were not balanced, the train would
sink into the ground or float upward, depending on the
direction of the greater force.
When a train moves along a straight track, the force from
the engine moves the train forward. The force of friction
between the wheels and the track is equal in size to the
engine’s force, but in the opposite direction. The forces
acting on the train are balanced. The train does not
accelerate, but it moves at a constant velocity. When
balanced forces act on an object, the motion is constant. The
object is either at rest—that is, completely still—or it is
moving at a constant velocity.
Forces and Newton’s Laws of Motion
Isaac Newton was an English scientist who lived in the
late 1600s. He developed three important rules about motion
called Newton’s laws of motion.
Newton’s First Law of Motion
13. Explain What is inertia?
14. Infer Before the crash,
why does the car move with
a constant velocity?
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Motion and Forces
Newton’s first law explains why safety belts can protect
passengers in a car from danger. Engineers who study car
crashes often use crash-test dummies as models of car
passengers. Because of inertia, crash-test dummies without
safety belts keep moving forward after a car crashes into
a wall. Before the crash, the car and the test dummies move
with a constant velocity. When the car crashes into the wall,
unbalanced forces act on the car, and it stops. However, the
dummies, which are not attached to the car, continue to
move with a constant velocity because of their inertia. They
continue to move until an unbalanced force changes their
motion. If that unbalanced force does not come from a
safety belt, it could come from the windshield.
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Reading Check
As you just read, when balanced forces act on an object,
the object’s motion is constant. According to Newton’s first
law of motion, if the net force acting on an object is zero, the motion
of the object does not change. Newton’s first law of motion
sometimes is called the law of inertia. Inertia is the tendency
of an object to resist a change in its motion.
Newton’s Second Law of Motion
As you have read, when an unbalanced force acts on an
object, the object accelerates. Newton’s second law of motion
states that acceleration of an object is equal to the net force exerted on
the object divided by the object’s mass. This means that the greater
the mass, the greater the force must be to accelerate the
object at the same rate. Acceleration is in the same direction
as net force. You can write Newton’s second law of motion
as an equation:
force (f )
acceleration (a) = ________
mass (m)
For example, think about a bowler exerting a force by
throwing a 4-kg bowling ball. A force of 80 N is needed to
cause a mass of 4 kg to accelerate to 20 m/s every second.
The greater the force is that the bowler exerts on the ball,
the faster the ball will accelerate as long as the mass of the
ball remains constant. If the bowler wanted to accelerate
a mass of 8 kg at the same rate (to 20 m/s every second), the
bowler would need a force of 160 N.
Reading Check
15. State What effect does
an increase in an object’s
mass have on the acceleration
of the object if net force
remains the same?
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Newton’s Third Law of Motion
Imagine a runner on a starting block. The runner’s feet
exert a downward force on the block. The block exerts an
upward force on the runner’s feet. When the runner pushes
on the block to start the race, the block pushes back, helping
the runner get a fast start. Newton’s third law of motion says
that for every action there is an equal and opposite reaction. In other
words, when one object exerts a force on a second object,
the second object exerts a force of equal size in the opposite
direction on the first object. These equal and opposite forces
are called force pairs. The runner’s feet exert a downward
force on the starting block, and the starting block exerts an
upward force on the runner’s feet. This is the force pair.
Force pairs are not the same as balanced forces. Balanced
forces act on the same object. Recall the figure of the train,
shown earlier in the lesson. The force from gravity and the
force from the train track are balanced and they act on the
same object—the train.
Think about Newton’s laws as you move through your
day. Together, the laws help you understand why objects
move as they do.
Reading Essentials
Reading Check
16. Contrast force pairs
and balanced forces.
Motion and Forces
143
Mini Glossary
air resistance: the frictional force between air and objects
moving through it
contact force: a push or a pull one object applies to another
Newton’s first law of motion: the law that states that if
the net force acting on an object is zero, the motion of the
object does not change
Newton’s second law of motion: the law that states that
object that is touching it
acceleration of an object is equal to the net force exerted on the
object divided by the object’s mass
force: a push or a pull on an object
friction: a contact force that resists the sliding motion of two
surfaces that are touching
Newton’s third law of motion: the law that states that for
every action there is an equal and opposite reaction
gravity: an attractive force that exists between all objects that
have mass
noncontact force: a force that one object applies to another
object without touching it
1. Review the terms and their definitions in the Mini Glossary. Write a sentence that
illustrates an example of one of Newton’s laws of motion.
2. Use terms from the lesson to fill in the missing bubbles in the concept map below.
magnetism
contact
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Types of Forces
kick
electrical force
What do you think
Reread the statements at the beginning of the
lesson. Fill in the After column with an A if you
agree with the statement or a D if you disagree.
Did you change your mind?
144
Motion and Forces
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