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KEY CONCEPT
Forces act in pairs.
BEFORE, you learned
NOW, you will learn
• A force is a push or a pull
• Increasing the force on
an object increases the
acceleration
• The acceleration of an object
depends on its mass and the
force applied to it
• How Newton’s third law
relates action/reaction pairs
of forces
• How Newton’s laws
work together
VOCABULARY
THINK ABOUT
Newton’s third law p. 57
How do jellyfish move?
Jellyfish do not have much control
over their movements. They drift
with the current in the ocean.
However, jellyfish do have some
control over their up-and-down
motion. By squeezing water out of
its umbrella-like body, the jellyfish shown here applies a force in one
direction to move in the opposite direction. If the water is forced
downward, the jellyfish moves upward. How can a person or an object
move in one direction by exerting a force in the opposite direction?
Newton’s third law relates action and
reaction forces.
COMBINATION NOTES
In your notebook, make an
outline and draw a diagram
about Newton’s third law.
Newton made an important observation that explains the motion
of the jellyfish. He noticed that forces always act in pairs.
Newton’s third law states that every time one object exerts a force
on another object, the second object exerts a force that is equal in
size and opposite in direction back on the first object. As the jellyfish
contracts its body, it applies a downward force on the water. The
water applies an equal force back on the jellyfish. It is this equal and
opposite force on the jellyfish that pushes it up. This is similar to
what happens when a blown-up balloon is released. The balloon
pushes air out the end, and the air pushes back on the balloon and
moves it forward.
check your reading
What moves the jellyfish through the water?
Chapter 2: Forces 57
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Action and Reaction Pairs
The force that is exerted on an object and the force that the object
exerts back are known together as an action/reaction force pair. One
force in the pair is called the action force, and the other is called the
reaction force. For instance, if the jellyfish pushing on the water is the
action force, the water pushing back on the jellyfish is the reaction
force. Likewise, if the balloon pushing the air backward is the action
force, the air pushing the balloon forward is the reaction force.
You can see many examples of action and reaction forces in the
world around you. Here are three:
•
•
•
You may have watched the liftoffs of the space shuttle on
television. When the booster rockets carrying the space shuttle
take off, their engines push fuel exhaust downward. The exhaust
pushes back on the rockets, sending them upward.
When you bang your toe into the leg of a table, the same amount
of force that you exert on the table is exerted back on your toe.
Action and reaction forces do not always result in motion. For
example, if you press down on a table, the table resists the push
with the same amount of force, even though nothing moves.
check your reading
Identify the action/reaction forces in each example
described above.
Newton’s Third Law
How do action and reaction forces compare?
SKILL FOCUS
Observing
PROCEDURE
1
With a partner, hook the two spring scales together.
MATERIALS
2
Pull gently on your spring scale while your partner holds but does not pull
on the other scale.
TIME
3
Observe and record the amount of force that is shown on your scale and on
your partner’s scale.
4
Both of you pull together. Observe the force shown on each scale.
WHAT DO YOU THINK?
• What happened to your partner’s force as your force increased?
• What happened when you both pulled?
• Explain why you think what you observed in each case happened.
CHALLENGE Can you think of a way to use the scales to show
Newton’s first or second law?
58 Unit 1: Motion and Forces
2 spring scales
15 minutes
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Action and Reaction Forces Versus Balanced Forces
Because action and reaction forces are equal and opposite, they may be
confused with balanced forces. Keep in mind that balanced forces act on
a single object, while action and reaction forces act on different objects.
If you and a friend pull on opposite sides of a backpack with the same amount of force, the backpack doesn’t move,
because the forces acting on it are balanced. In this case, both forces
are exerted on one object—the backpack.
Balanced Forces
As you drag a heavy backpack across a floor, you
can feel the backpack pulling on you with an equal amount of force.
The action force and the reaction force are acting on two different
things—one is acting on the backpack, and the other is acting on you.
Action and Reaction
The illustration below summarizes Newton’s third law. The girl
exerts an action force on the boy by pushing him. Even though the
boy is not trying to push the girl, an equal and opposite reaction force
acts upon the girl, causing her to move as well.
Newton’s Third Law
When one object exerts a force on another object, the second
object exerts an equal and opposite force on the first object.
1
One Skater Pushes
reaction
force
2
Both Skaters Move
action
force
The action force from the girl sets the
boy in motion.
Even though the boy does not do anything, the
reaction force from him sets the girl in motion as well.
How does the direction of the force on the girl
relate to her motion?
Chapter 2: Forces 59
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Newton’s Three Laws of Motion
All three of Newton’s laws work together to help describe
how an object will move.
Newton’s First Law
Newton’s Second Law
Newton’s Third Law
action
force
acceleration
force of gravity
This kangaroo has jumped,
setting itself in motion. If no
other forces acted on it, the
kangaroo would continue to
move through the air with the
same motion. Instead, the
force of gravity will bring this
kangaroo back to the ground.
The large kangaroo does not
have as much acceleration as a
less massive kangaroo would if
it used the same force to jump.
However, the more massive
kangaroo can increase its acceleration by increasing the force
of its jump.
A kangaroo applies an action
force on the ground with its
powerful back legs. The reaction force from the ground
can send the kangaroo as far
as 8 meters (26 ft) through
the air.
Common Name: Red kangaroo
Scientific Name: Macropus rufus
Home: Australia
What forces are involved in
a kangaroo jump?
60 Unit 1: Motion and Forces
reaction
force
Top Speed: 65 km/h (40 mi/h)
Maximum Leap: 8 m (26 ft)
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Newton’s three laws describe and
predict motion.
Newton’s three laws can explain the motion of almost any object,
including the motion of animals. The illustrations on page 60 show
how all three of Newton’s laws can be used to describe how kangaroos
move. The three laws are not independent of one another; they are
used together to explain the motion of objects.
RESOURCE CENTER
CLASSZONE.COM
Find out more about
Newton’s laws of
motion.
You can use the laws of motion to explain how other animals
move as well. For example, Newton’s laws explain why a squid moves
forward while squirting water out behind it. These laws also explain
that a bird is exerting force when it speeds up to fly away or when it
changes its direction in the air.
You can also use Newton’s laws to make predictions about
motion. If you know the force acting upon an object, then you can
predict how that object’s motion will change. For example, if you
want to send a spacecraft to Mars, you must be able to predict exactly
where Mars will be by the time the spacecraft reaches it. You must
also be able to control the force on your spacecraft so that it will
arrive at the right place at the right time.
COMBINATION NOTES
Make an outline and
draw a diagram showing
how all three of Newton’s
laws apply to the motion
of one object.
Knowing how Newton’s three laws work together can also help you
win a canoe race. In order to start the canoe moving, you need to apply
a force to overcome its inertia. Newton’s second law might affect your
choice of canoes, because a less massive canoe is easier to accelerate
than a more massive one. You can also predict the best position for your
paddle in the water. If you want to move straight ahead, you push
backward on the paddle so that the canoe moves forward. Together,
Newton’s laws can help you explain and predict how the canoe, or any
object, will move.
KEY CONCEPTS
CRITICAL THINKING
1. Identify the action/reaction
force pair involved when you
catch a ball.
4. Apply A man pushes on a
wall with a force of 50 N.
What are the size and the
direction of the force that the
wall exerts on the man?
2. Explain the difference between
balanced forces and
action/reaction forces.
3. How do Newton’s laws of
motion apply to the motion
of an animal, such as a cat
that is running?
5. Evaluate Jim will not help
push a heavy box. He says, “My
force will produce an opposite
force and cancel my effort.”
Evaluate Jim’s statement.
CHALLENGE
6. Calculate Suppose you are
holding a basketball while
standing still on a skateboard.
You and the skateboard have
a mass of 50 kg. You throw
the basketball with a force of
10 N. What is your acceleration before and after you
throw the ball?
Chapter 2: Forces 61