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5Newtons Third Law of Motion

Main topics:

Forces and Interaction

Newtons Third Law of

Motion

Summary of Newtons

Three Laws

The heavy weight champion can hit

the massive bag with considerable

force. But with the same punch hecan only exert a tiny force on the

tissue paper in midair. Why is this?

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Forces and Interaction

A force is not a thing in

itself but makes up an

interactionbetween one thingand another.

These forces are equal in

magnitude and opposite in

direction and comprise asingle interaction.

When you lean against a

wall, you exert a force on

the wall. The wall

simultaneously exerts an

equal and opposite force

on you. Hence, you dont

topple over.

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An Example of the Boxer

He can hit the massive bag

with considerable force.

But with the same punch he

can exert only a tiny force

on the tissue paper in

midair.

The boxers fist can only

exert as much force on the

tissue paper as the tissue

paper can exert on the fist. The fist cant exert any

force at all unless what is

being hit exerts the same

amount of force back.

An interaction requires a

pairof forces acting on two

objects.

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Other Examples of Forces and

Interactions

In the interactionbetween the hammer and

the stake, each exerts the

same amount of force on

the other.

The impact forces betweenthe blue and yellow balls

move the yellow ball and

stop the blue ball.

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Which Exerts the Force and

Which Receives the Force?j Isaac Newtons answer:

Neither force has to be identified as exerter or

receiver. Both objects must be treated equally. (Think

about the hammer and stake.)

Newtons Third Law of Motion

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Newtons Third Law of Motion

Newtons third law states:

Whenever one object exerts a force on a secondobject, the second object exerts an equal and

opposite force on the first. In any reaction there is an action and reaction pair of

forces that are equal in magnitude and opposite indirection.

Neither force exists without the other-forces come inpairs, one action and the other reaction.

The action and reaction pair of forces makes up oneinteraction between two things.

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More Examples of Newtons Third Law

of Motion

Action and reaction forces. Note that when action is

A exerts force on B, the reaction is then simply

B exerts force on A.

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Action and Reaction on Different

Masses

The Earth is pulled up by theboulder with just as much force as

the boulder is pulled downward by

the Earth.

Why do you only notice the object

is accelerating but you dont notice

the acceleration of the earth? -

Newtons 2nd law of motion!

Which falls toward

the other, A or B? Dothe accelerations of

each relate to their

relative masses?

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More Examples-Action and

Reaction on Different Masses

The force exerted against the

recoiling rifle is just as great

as the force that drives the

bullet. Why then, does the

bullet accelerate more than therifle?

Use Newtons 2nd Law:

We must not only

consider theforce (net

force) applied but also themass involved.

The acceleration of the

bullet is given by F/m = a

The acceleration of therecoiling rifle is F/m = a

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Summaries of Newtons Three Laws of

Motionj

The first law: An object at rest tends to remain at rest; an object in motion tends toremain in motion at constant speed along a straight line.

The tendency of objects to resist change in motion is called inertia.

Mass is a measure of inertia.

Objects will undergo changes in motion in the presence of a net force.

j The second law: When a force acts on an object, the object will accelerate.

The acceleration is directly proportional to the net force and inverselyproportional to the mass, a = F/m.

j The third law:

Whenever one object exerts a force on a second object, the second objectexerts an equal and opposite force on the first.

Forces come in pairs, one action and the other reaction, both of whichcomprise the interactionbetween one object and the other.

Action and reaction always act on differentobjects.

Neither force exists without the other.

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Homework

jChapter 5, Page 88-89, Exercises: #4, 15,

24.

jThe above problems are assigned from the10th edition of the textbook by Hewitt.