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# Lec9 - Physics 2A Olga Dudko UCSD Physics Lecture 9 Today...

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Today: Newton’s Third Law. Donkey-and-cart dilemma Applying Newton’s Laws. Physics 2A Olga Dudko UCSD Physics Lecture 9

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Newton’s Third Law Newton Newton s s Third Law: Third Law: If an object A exerts a force on object B, then object B exerts an oppositely directed force of equal magnitude on A. • Also stated as: For every action, there is an equal, but opposite reaction. • Let’s look at a game of Tug-Of-War.
Clicker Question Who wins a game of Tug-Of-War between a 80kg person and a 60kg person? A) Newton’s Third Law tells that neither can win, because the force between them is equal and opposite. B) You can’t tell until you draw your force diagram for this situation. C) The 80kg person, because the 80kg person will always exert a greater force than the 60kg person. D) The 60kg person, because the force exerted by this much mass is greater than the force exerted by the 80kg person.

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Tug-of-war force diagrams • Draw a force diagram for each person separately: • The winner of the tug-of-war contest is not who is the strongest (F A on B = F B on A ), but whoever had the most friction with the ground. 80kg F normal, ground on 80kg F gravity, Earth on 80kg F friction, ground on 80kg F tension, 60kg on 80kg 60kg F normal, ground on 60kg F gravity, Earth on 60kg F friction, ground on 60kg F tension, 80kg on 60kg
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