Chapter 7. Newton’s Third Law - Physics & Astronomy

Chapter 7. Newton’s Third LawChapter 7. Newton’s Third Law

The harder one sumo

wrestler pushes, the harder

the other pushes back.

Newton’s third law describes

how two objects interact

with each other.

Chapter Goal: To use

Newton’s third law to

understand interacting

objects.

Topics:

• Interacting Objects

• Analyzing Interacting Objects

• Newton’s Third Law

Chapter 7. Newton’s Third LawChapter 7. Newton’s Third Law

• Ropes and Pulleys

• Examples of Interacting-Object Problems

Interacting Objects

If object A exerts a force on object B, then object B

exerts a force on object A. The pair of forces, as shown,

is called an action/reaction pair.

The action-reaction pair never occur on the same object !

Newton’s Third Law:

100T N====

1 2100T T N= == == == =

1Trrrr

2Trrrr

The Massless String Approximation

Often in physics and engineering problems the mass of the

string or rope is much less than the masses of the objects

that it connects. In such cases, we can adopt the following

massless string approximation:

EXAMPLE 7.6 Comparing two tensions

QUESTION:

1100T N====

Find tension of rope 2

m kg====

No frictionSystem A:

2,ATrrrr

Anrrrr

, 2, 1A net A A A AF T n w T m a= + + + == + + + == + + + == + + + =

r r rr r rr r rr r rr r rr r rr r rr r r

2?T ====

Awrrrr , 2, 1A net A A A A

F T n w T m a= + + + == + + + == + + + == + + + =

1 2,A AT T m a− =− =− =− =

System B:

2,BTrrrr

Bwrrrr

Bnrrrr

, 2, 1B net B B B BF T n w m a= + + == + + == + + == + + =

r rr rr rr r r r rr r rr r rr r r

2,B BT m a====

1100T N====

Find tension of rope 2

m kg====

No frictionSystem A:

2,ATrrrr

1 2,A AT T m a− =− =− =− =

2?T ====

System B: 2,BTrrrr

2,B BT m a====

2, 2, 2B AT T T= == == == =Newton’s Third Law:

1 2 AT T m a− =− =− =− =

2 BT m a====

m m====

4 10080

m TT N

m m= = == = == = == = =

��

Example: The 10.2 kg block is held in place by the massless rope passing over

two massless, frictionless pulleys. Find the tensions to and the

magnitude of force F. 1

The same rope:

2 3 5T T T= == == == =

1Trrrr

Equilibrium:

110.2 10 102T w N= ≈ == ≈ == ≈ == ≈ =��

1Trrrr

3Trrrr

2Trrrr

Equilibrium:

1 3 2 22T T T T= + == + == + == + =

2 3 551

TT N T T= = = == = = == = = == = = =

5Trrrr

Equilibrium:

551F T N= == == == =

2 3 551T T T N= = == = == = == = =

4Trrrr

1102T N====

51F N====

2Trrrr

4 2 3 5153T T T T N= + + == + + == + + == + + =

Equilibrium:

5Trrrr

3Trrrr

EXAMPLE 7.7 Mountain climbing

QUESTION:

Chapter 7. Summary SlidesChapter 7. Summary Slides

General Principles

Important Concepts

Applications

Chapter 7. Chapter 7. QuestionsQuestions

A fishing line of negligible mass lifts a fishupward at constant speed. The line and the fish are the system, the fishing pole is part of the environment. What, if anything, is wrong with the free-body diagrams?

A. The force of the pole on the fish is missing.

B. The force of gravity on the line is

B. The force of gravity on the line is missing.

C. The gravitational force and the tension force are incorrectly identified as an action/reaction pair.

D. There should be only one force on the fish.

E. There is nothing wrong with the free-body diagrams.

A fishing line of negligible mass lifts a fishupward at constant speed. The line and the fish are the system, the fishing pole is part of the environment. What, if anything, is wrong with the free-body diagrams?

A. The force of the pole on the fish is missing.

B. The force of gravity on the line is

B. The force of gravity on the line is missing.

C. The gravitational force and the tension force are incorrectly identified as an action/reaction pair.

D. There should be only one force on the fish.

E. There is nothing wrong with the free-body diagrams.

Car B is stopped for a red light. Car A, which has the same mass as car B, doesn’t see the red light and runs into the back of B. Which of the following statements is true?

A. A exerts a larger force on B than B exerts on A.

A. A exerts a larger force on B than B exerts on A.B. A exerts a force on B but B doesn’t exert a force

on A.C. B exerts a force on A but A doesn’t exert a force

on B.D. B exerts a larger force on A than A exerts on B.E. B exerts the same amount of force on A as A

exerts on B.

Car B is stopped for a red light. Car A, which has the same mass as car B, doesn’t see the red light and runs into the back of B. Which of the following statements is true?

A. A exerts a larger force on B than B exerts on A.

A. A exerts a larger force on B than B exerts on A.B. A exerts a force on B but B doesn’t exert a force

on A.C. B exerts a force on A but A doesn’t exert a force

on B.D. B exerts a larger force on A than A exerts on B.E. B exerts the same amount of force on A as A

exerts on B.

Boxes A and B are sliding to the right across a frictionless table. The hand H is slowing them down. The mass of A is larger than the mass of B. Rank in order, from largest to smallest, the horizontal forces on A, B, and H.

A. FH on B = FH on A > FA on B

B. FB on H = FH on B = FA on B = FB on A

C. FB on H = FH on B < FA on B = FB on A

D. FB on H = FH on B < FA on B = FB on A

Boxes A and B are sliding to the right across a frictionless table. The hand H is slowing them down. The mass of A is larger than the mass of B. Rank in order, from largest to smallest, the horizontal forces on A, B, and H.

A. FH on B = FH on A > FA on B

B. FB on H = FH on B = FA on B = FB on A

C. FB on H = FH on B < FA on B = FB on A

D. FB on H = FH on B < FA on B = FB on A

A. Equal to

B. Greater than

All three 50 kg blocks are at rest. Is the tension in rope 2 greater than, less than, or equal to the tension in rope 1?

B. Greater than

C. Less than

A. Equal to

B. Greater than

All three 50 kg blocks are at rest. Is the tension in rope 2 greater than, less than, or equal to the tension in rope 1?

B. Greater than

C. Less than

A small car is pushing a larger truck that has a dead battery. The mass of the truck is larger than the mass of thecar. Which of the following statements is true?A. The truck exerts a larger force on the car than the car exerts

A. The truck exerts a larger force on the car than the car exerts on the truck.

B. The truck exerts a force on the car but the car doesn’t exert a force on the truck.

C. The car exerts a force on the truck but the truck doesn’t exert a force on the car.

D. The car exerts a larger force on the truck than the truck exerts on the car.

E. The car exerts the same amount of force on the truck as the truck exerts on the car.

A small car is pushing a larger truck that has a dead battery. The mass of the truck is larger than the mass of thecar. Which of the following statements is true?A. The truck exerts a larger force on the car than the car exerts

A. The truck exerts a larger force on the car than the car exerts on the truck.

B. The truck exerts a force on the car but the car doesn’t exert a force on the truck.

C. The car exerts a force on the truck but the truck doesn’t exert a force on the car.

D. The car exerts a larger force on the truck than the truck exerts on the car.

E. The car exerts the same amount of force on the truck as the truck exerts on the car.

Chapter 7. Newton’s Third Law - Physics & Astronomy

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