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NEWTON'S THIRD LAW OF MOTION
• This lesson will introduce you to the last of three laws of dynamics
• These laws are the fundamental laws of Dynamics. With them, you can predict how an object will move when under the influence of forces.
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By the end of this lesson, you will be able to:
What are we going to cover today? Explain why
forces exist in pairs
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State Newton’s 3rd Law of MotionExplain Newton’s 3rd Law of Motion
Homework
Solve problems with the 3rd Law
FORCES EXIST IN PAIRSS
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• Newton discovered that when two objects interact, two forces will always be involved• These two forces are called action and reaction forces
Can you identify the action – reaction forces in this diagram?
CofG
Fn
Fg
No contact = no force
Fn
Fa
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For every action force that object A exerts on object B, object B exerts a reaction force that is equal in magnitude, but opposite in direction on object A•Newton’s 3rd law of
motion tells us that action-reaction forces are
•equal in magnitude
•opposite in direction
AonB BonAF F
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s NEWTON’S THIRD LAW PROBLEMS
Ex 1.Draw a free-body diagram of a 2.5 kg book
resting on a tableSol’n
2.5 kg
Force of table on bookFN = 25 N [U]
Force of gravity on bookFg = 25 N [D]
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1. If the table pushes on the book with 25 N [U], then the book pushes on the table with 25 N [D]
There are two reaction forces, because there are two Forces on the free-body diagrams.
2.If the Earth pulls on the book with a force of 25 N [D], then the book must pull on the Earth with 25 N[U]
That’s Right. The book pulls on the earth with an equal force, just opposite in direction.
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s APPLICATIONS OF THE THIRD LAW
1. Walking
When you walk, you apply a force to the earth (the action force). - In this case down and backwards- By Newton’s 3rd law the Earth will push back on you (the reaction force) up and forward. The forces are equal
in magnitude, but opposite in direction.
2. Swimming
When you swim, you apply a force to the water (the action force). By Newton’s 3rd law the water will push back on you (the reaction force). The forces are equal in magnitude, but opposite in direction.
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Suppose you push on an object
attempting to move it.
The reaction force is the object will
push back on you.
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sQuestion: If the force of the object pushing back on you is always equal & opposite, then how are you able to
move it (sometimes)?
Answer: Because they act on different objects.
Question: But don’t action - reaction
forces cancel each other out causing
Fnet = 0?
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Answer: They do cancel each other out!
But they don’t cancel out all the other
forces…
To understand this, lets Draw the FBD of each object
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FaFf
FN
Fg
This is the action force,
the applied force by the boy
on the man.
Fg
Fa
FNThis is the reaction force, the force which the larger object pushes back on the kid
Ff
We are looking at Fa in both of these FBD’s
Question: Why should we expect to see acceleration? Question: But how does that happen if all these forces are equal and opposite?
PRACTICE QUESTIONS
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SMART Response Question
1.
a. 67. Nb. 6.7 Nc. .67 Nd. .067 Ne. 6.7 x 10-7 N
HOMEWORKS
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