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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
• 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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
The action-reaction pair never occur on the same object !
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Newton’s Third Law:
100T N====
1 2100T T N= == == == =
Trrrr
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
7
1Trrrr
2Trrrr
The Massless String Approximation
Often in physics and engineering problems the mass of the
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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:
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
EXAMPLE 7.6 Comparing two tensions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
EXAMPLE 7.6 Comparing two tensions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
EXAMPLE 7.6 Comparing two tensions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
1100T N====
Find tension of rope 2
1A
m kg====
4B
m kg====
No frictionSystem A:
2,ATrrrr
Anrrrr
arrrr
x
, 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 ====
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
13
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
arrrr
x
, 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
1A
m kg====
4B
m kg====
No frictionSystem A:
2,ATrrrr
arrrr
1 2,A AT T m a− =− =− =− =
2?T ====
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
14
System B: 2,BTrrrr
arrrr
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====
1
A B
Ta
m m====
++++
1
2
4 10080
5
B
A B
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
T5
T
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
15
The same rope:
2 3 5T T T= == == == =
2 3 5T T T= == == == =
1Trrrr
wrrrr
Equilibrium:
110.2 10 102T w N= ≈ == ≈ == ≈ == ≈ =����
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
16
1Trrrr
3Trrrr
2Trrrr
Equilibrium:
1 3 2 22T T T T= + == + == + == + =
1
2 3 551
2
TT N T T= = = == = = == = = == = = =
5Trrrr
Equilibrium:
551F T N= == == == =
2 3 551T T T N= = == = == = == = =
4Trrrr
1102T N====
51F N====
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
17
2Trrrr
4 2 3 5153T T T T N= + + == + + == + + == + + =
Equilibrium:
5Trrrr
3Trrrr
EXAMPLE 7.7 Mountain climbing
QUESTION:
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
EXAMPLE 7.7 Mountain climbing
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
EXAMPLE 7.7 Mountain climbing
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
EXAMPLE 7.7 Mountain climbing
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
EXAMPLE 7.7 Mountain climbing
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Chapter 7. Summary SlidesChapter 7. Summary Slides
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Chapter 7. Summary SlidesChapter 7. Summary Slides
General Principles
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
General Principles
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Important Concepts
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Important Concepts
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Applications
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Applications
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Chapter 7. Chapter 7. QuestionsQuestions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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?
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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?
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
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.