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# Lect_09 - Lecture 9 Newton's Third and Free Body Diagrams I...

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I SOMETIMES WISH NEWTON HAD STOPPED AT TWO LAWS. Lecture 9 : Newton’s Third and Free Body Diagrams Newton Newton’ s Third Law s Third Law For every force, or action there is an equal but opposite force, or reaction. Forces ALWAYS happen in pairs. BA AB F F G G = Newton Newton’ s third law: Action and Reaction Example: Gravitation. You attract the Earth! F g you,Earth = F g Earth, you F g you,Earth F g Earth, you But the acceleration that this produces on the Earth is nothing to be too proud of…) , m/s 10 kg 10 6 ) m/s 10 )( kg 70 ( weight your 2 22 24 2 Earth × = M a EXAMPLES N person ,wall N wall, person Person leaning against the wall Person leaning against the wall

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On exhaust particles, by rocket On rocket, by exhaust particles, On bullet, by gun On gun, by bullet On Saturn, by Mars On Mars, by Saturn On the car, by magnet On magnet, by car Rocket Ship Newton’s third law implies that if a rocket accelerates forwards, something must be pushed backwards. In outer space, there isn’t much else around besides its own fuel. rocket DEMO: What if roads where not properly fixed to the ground? Two carts are put back-to-back on a track. Cart A has a spring- loaded piston; cart B, which has twice the mass of cart A, is entirely passive. When the piston is released, it pushes against cart B, and the carts move apart. Which of the two forces exerted by the two carts on each other has a larger magnitude? 1. The force exerted by A. 2. The two forces have equal magnitude. 3. The force exerted by B. A B ACT: Carts with spring It’s a third law pair!! DEMO: Skateboards A Book on a Table Normal on book by table
Normal on book by table Normal on table by book Book on Table – The full story Normal force between book and table N BT = – N TB W BE N BT N TE N TB W TE W ET W EB N ET The book does not accelerate W BE + N BT =0

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