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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s NEWTON’S THIRD LAW OF MOTION

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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s Ex 2.State the reaction forces to the book in ex 1.Sol’n

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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