5
Main topics:
• Forces and Interaction
The heavy weight champion can hit the massive bag with considerable force. But with the same punch he can only exert a tiny force on the tissue paper in midair. Why is this?
• Newton’s Third Law of
Motion
• Summary of Newton’s
Three Laws
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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.
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• A force is not a thing in itself but makes up an interaction between one thing and another.
• These forces are equal in magnitude and opposite in direction and comprise a single interaction .
2

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 boxer’s fist can only exert as much force on the tissue paper as the tissue paper can exert on the fist.
• 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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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.
4

 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
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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.
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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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Which falls toward the other, A or B? Do the accelerations of each relate to their relative masses?
• The Earth is pulled up by the boulder 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 don’t notice the acceleration of the earth? -
Newton’s 2nd law of motion!
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Use Newton’s 2nd Law:
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 the rifle?
• We must not only consider the force (net force) applied but also the mass involved.
• The acceleration of the bullet is given by F/m = a
The acceleration of the recoiling rifle is F/ m
= a
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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.
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
Chapter 5, Page 88-89, Exercises: #4, 15,
24.

The above problems are assigned from the
10 th edition of the textbook by Hewitt.
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