chapter 7. newton’s third law - physics & astronomy
TRANSCRIPT
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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.
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with each other.
Chapter Goal: To use
Newton’s third law to
understand interacting
objects.
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Topics:
• Interacting Objects
• Analyzing Interacting Objects
• Newton’s Third Law
Chapter 7. Newton’s Third LawChapter 7. Newton’s Third Law
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• Ropes and Pulleys
• Examples of Interacting-Object Problems
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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.
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The action-reaction pair never occur on the same object !
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Newton’s Third Law:
100T N====
1 2100T T N= == == == =
Trrrr
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7
1Trrrr
2Trrrr
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The Massless String Approximation
Often in physics and engineering problems the mass of the
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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:
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EXAMPLE 7.6 Comparing two tensions
QUESTION:
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EXAMPLE 7.6 Comparing two tensions
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EXAMPLE 7.6 Comparing two tensions
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EXAMPLE 7.6 Comparing two tensions
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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 ====
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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====
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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 ====
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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= = == = == = == = =
++++
����
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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
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15
The same rope:
2 3 5T T T= == == == =
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2 3 5T T T= == == == =
1Trrrr
wrrrr
Equilibrium:
110.2 10 102T w N= ≈ == ≈ == ≈ == ≈ =����
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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= == == == =
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2 3 551T T T N= = == = == = == = =
4Trrrr
1102T N====
51F N====
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17
2Trrrr
4 2 3 5153T T T T N= + + == + + == + + == + + =
Equilibrium:
5Trrrr
3Trrrr
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EXAMPLE 7.7 Mountain climbing
QUESTION:
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EXAMPLE 7.7 Mountain climbing
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EXAMPLE 7.7 Mountain climbing
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EXAMPLE 7.7 Mountain climbing
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EXAMPLE 7.7 Mountain climbing
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Chapter 7. Summary SlidesChapter 7. Summary Slides
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Chapter 7. Summary SlidesChapter 7. Summary Slides
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General Principles
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General Principles
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Important Concepts
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Important Concepts
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Applications
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Applications
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Chapter 7. Chapter 7. QuestionsQuestions
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Chapter 7. Chapter 7. QuestionsQuestions
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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
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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
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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?
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B. Greater than
C. Less than
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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?
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B. Greater than
C. Less than
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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
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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.
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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
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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.