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Dr. Jie ZouPHY Newton’s Third Law of Motion Main topics: Forces and Interaction Newton’s Third Law of Motion Summary of Newton’s Three Laws 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?

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Dr. Jie ZouPHY Forces and Interaction 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. 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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Dr. Jie ZouPHY 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 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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Dr. Jie ZouPHY 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. The impact forces between the blue and yellow balls move the yellow ball and stop the blue ball.

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Dr. Jie ZouPHY 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

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Dr. Jie ZouPHY 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.

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Dr. Jie ZouPHY 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.”

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Dr. Jie ZouPHY 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. 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! Which falls toward the other, A or B? Do the accelerations of each relate to their relative masses?

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Dr. Jie ZouPHY 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 the rifle? Use Newton’s 2nd Law: 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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Dr. Jie ZouPHY 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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Dr. Jie ZouPHY Homework 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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