5 Newton’s Third Law of Motion
Main topics:
• Forces and Interaction
• Newton’s Third Law of
The heavy weight champion can hit Motion
the massive bag with considerable • Summary of Newton’s
force. But with the same punch he Three Laws
can only exert a tiny force on the
tissue paper in midair. Why is this?
Dr. Jie Zou
PHY 1071
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Forces and Interaction
• A force is not a thing in
itself but makes up an
interaction between one thing
and another.
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 don’t
topple over.
Dr. Jie Zou
• These forces are equal in
magnitude and opposite in
direction and comprise a
single interaction.
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An Example of the Boxer
• The boxer’s fist can only
exert as much force on the
tissue paper as the tissue
paper can exert on the fist.
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.
Dr. Jie Zou
PHY 1071
• The fist can’t exert any
force at all unless what is
being hit exerts the same
amount of force back.
• An interaction requires a
pair of forces acting on two
objects.
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Other Examples of Forces and
Interactions
In the interaction
between the hammer and
the stake, each exerts the
same amount of force on
the other.
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The impact forces between
the 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?
♦ Isaac Newton’s answer:
– Neither force has to be identified as “exerter” or
“receiver”.
– Both objects must be treated equally. (Think
about the hammer and stake.)
Newton’s Third Law of Motion
Dr. Jie Zou
PHY 1071
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Newton’s Third Law of Motion
Newton’s third law states:
Whenever one object exerts a force on a second
object, 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 in
direction.
– Neither force exists without the other-forces come in
pairs, one action and the other reaction.
– The action and reaction pair of forces makes up one
interaction between two things.
Dr. Jie Zou
PHY 1071
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More Examples of Newton’s 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.”
Dr. Jie Zou
PHY 1071
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Action and Reaction on Different
Masses
• The Earth is pulled up by the
boulder with just as much force as
the boulder is pulled downward by
the Earth.
Which falls toward
the other, A or B?
Do the accelerations
of each relate to their
relative masses?
• Why do you only notice the object
is accelerating but you don’t notice
the acceleration of the earth? Newton’s 2nd law of motion!
Dr. Jie Zou
PHY 1071
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More Examples-Action and
Reaction on Different Masses
Use Newton’s 2nd Law:
• We must not only
consider the force (net
force) applied but also the
mass involved.
The force exerted against the
recoiling rifle is just as great
• The acceleration of the
as the force that drives the
bullet. Why then, does the
bullet is given by F/m =
bullet accelerate more than the
rifle?
The acceleration of the
a
•
recoiling rifle is F/m = a
Dr. Jie Zou
PHY 1071
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Summaries of Newton’s Three Laws of
Motion
♦ The first law:
– An object at rest tends to remain at rest; an object in motion tends to
remain 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.
♦ The second law:
– When a force acts on an object, the object will accelerate.
– The acceleration is directly proportional to the net force and inversely
proportional to the mass, a = F/m.
♦ The third law:
– Whenever one object exerts a force on a second object, the second object
exerts an equal and opposite force on the first.
– Forces come in pairs, one action and the other reaction, both of which
comprise the interaction between one object and the other.
– Action and reaction always act on different objects.
– Neither force exists without the other.
Dr. Jie Zou
PHY 1071
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Homework
♦ Chapter 5, Page 88-89, Exercises: #4, 15,
24.
♦ The above problems are assigned from the
10th edition of the textbook by Hewitt.
Dr. Jie Zou
PHY 1071
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