Chapter 11: Forces 361

VOCABULARY

Newton’s third law p. 361

BEFORE, you learned

• A force is a push or a pull• Increasing the force on

an object increases theacceleration

• The acceleration of an objectdepends on its mass and theforce applied to it

NOW, you will learn

• How Newton’s third law relates action/reaction pairs of forces

• How Newton’s laws work together

THINK ABOUT

How do jellyfish move?

Jellyfish do not have much controlover their movements. They driftwith the current in the ocean.However, jellyfish do have somecontrol over their up-and-downmotion. By squeezing water out ofits umbrella-like body, the jellyfish shown here applies a force in onedirection to move in the opposite direction. If the water is forceddownward, the jellyfish moves upward. How can a person or an objectmove in one direction by exerting a force in the opposite direction?

Newton’s third law relates action andreaction forces.

Newton made an important observation that explains the motion of the jellyfish. He noticed that forces always act in pairs.

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 jellyfishcontracts its body, it applies a downward force on the water. The water applies an equal force back on the jellyfish. It is this equal andopposite force on the jellyfish that pushes it up. This is similar to what happens when a blown-up balloon is released. The balloonpushes air out the end, and the air pushes back on the balloon andmoves it forward.

check your reading What moves the jellyfish through the water?

Newton’s third law

KEY CONCEPT

Forces act in pairs.

COMBINATION NOTESIn your notebook, make anoutline and draw a diagramabout Newton’s third law.

Action and Reaction Pairs The force that is exerted on an object and the force that the objectexerts back are known together as an action/reaction force pair. Oneforce in the pair is called the action force, and the other is called thereaction force. For instance, if the jellyfish pushing on the water is theaction force, the water pushing back on the jellyfish is the reactionforce. Likewise, if the balloon pushing the air backward is the actionforce, the air pushing the balloon forward is the reaction force.

You can see many examples of action and reaction forces in theworld around you. Here are three:

• You may have watched the liftoffs of the space shuttle on television. When the booster rockets carrying the space shuttletake 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 amountof force that you exert on the table is exerted back on your toe.

• Action and reaction forces do not always result in motion. Forexample, if you press down on a table, the table resists the pushwith the same amount of force, even though nothing moves.

check your reading Identify the action/reaction forces in each example described above.

How do action and reaction forces compare?PROCEDURE

With a partner, hook the two spring scales together.

Pull gently on your spring scale while your partner holds but does not pull on the other scale.

Observe and record the amount of force that is shown on your scale and onyour partner’s scale.

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?

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3

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Newton’s Third LawNewton’s Third LawSKILL FOCUSObserving

MATERIALS2 spring scales

TIME15 minutes

362 Unit 3: Motion and Forces

Chapter 11: Forces 363

Action and Reaction Forces Versus Balanced ForcesBecause action and reaction forces are equal and opposite, they may beconfused with balanced forces. Keep in mind that balanced forces act ona single object, while action and reaction forces act on different objects.

Balanced Forces If you and a friend pull on opposite sides of a back-pack with the same amount of force, the backpack doesn’t move,because the forces acting on it are balanced. In this case, both forcesare exerted on one object—the backpack.

Action and Reaction As you drag a heavy backpack across a floor, youcan feel the backpack pulling on you with an equal amount of force.The action force and the reaction force are acting on two differentthings—one is acting on the backpack, and the other is acting on you.

The illustration below summarizes Newton’s third law. The girlexerts an action force on the boy by pushing him. Even though theboy is not trying to push the girl, an equal and opposite reaction forceacts upon the girl, causing her to move as well.

When one object exerts a force on another object, the secondobject exerts an equal and opposite force on the first object.

Newton’s Third Law

One Skater Pushes1 Both Skaters Move2

How does the direction of the force on the girl relate to her motion?

reactionforce

actionforce

Even though the boy does not do anything, the reaction force from him sets the girl in motion as well.

The action force from the girl sets theboy in motion.

Newton’s Three Laws of Motion

All three of Newton’s laws work together to help describehow an object will move.

This kangaroo has jumped,setting itself in motion. If noother forces acted on it, thekangaroo would continue tomove through the air with thesame motion. Instead, theforce of gravity will bring thiskangaroo back to the ground.

The large kangaroo does nothave as much acceleration as aless massive kangaroo would ifit used the same force to jump.However, the more massivekangaroo can increase its accel-eration by increasing the forceof its jump.

A kangaroo applies an actionforce on the ground with itspowerful back legs. The reac-tion force from the groundcan send the kangaroo as faras 8 meters (26 ft) throughthe air.

accelerationforce of gravity

Common Name: Red kangaroo

Scientific Name: Macropus rufus

Home: Australia

Top Speed: 65 km/h (40 mi/h)

Maximum Leap: 8 m (26 ft)

364 Unit 3: Motion and Forces

What forces are involved ina kangaroo jump?

action force

reaction force

Newton’s First Law Newton’s Third LawNewton’s Second Law

Chapter 11: Forces 365

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 364 showhow all three of Newton’s laws can be used to describe how kangaroosmove. The three laws are not independent of one another; they areused together to explain the motion of objects.

You can use the laws of motion to explain how other animalsmove as well. For example, Newton’s laws explain why a squid movesforward while squirting water out behind it. These laws also explainthat a bird is exerting force when it speeds up to fly away or when itchanges 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 canpredict how that object’s motion will change. For example, if you want to send a spacecraft to Mars, you must be able to predict exactlywhere 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 willarrive at the right place at the right time.

Knowing how Newton’s three laws work together can also help youwin a canoe race. In order to start the canoe moving, you need to applya force to overcome its inertia. Newton’s second law might affect yourchoice of canoes, because a less massive canoe is easier to acceleratethan a more massive one. You can also predict the best position for yourpaddle in the water. If you want to move straight ahead, you pushbackward on the paddle so that the canoe moves forward. Together,Newton’s laws can help you explain and predict how the canoe, or anyobject, will move.

KEY CONCEPTS1. Identify the action/reaction

force pair involved when youcatch a ball.

2. Explain the difference betweenbalanced forces andaction/reaction forces.

3. How do Newton’s laws ofmotion apply to the motion of an animal, such as a catthat is running?

CRITICAL THINKING4. Apply A man pushes on a

wall with a force of 50 N.What are the size and thedirection of the force that thewall exerts on the man?

5. Evaluate Jim will not helppush a heavy box. He says, “Myforce will produce an oppositeforce and cancel my effort.”Evaluate Jim’s statement.

CHALLENGE6. Calculate Suppose you are

holding a basketball whilestanding still on a skateboard.You and the skateboard have a mass of 50 kg. You throwthe basketball with a force of10 N. What is your accelera-tion before and after youthrow the ball?

COMBINATION NOTESMake an outline anddraw a diagram showinghow all three of Newton’slaws apply to the motionof one object.

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