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Warm-up: Identify the action force and the corresponding reaction force for each event: A fish swims through water. A fish swims through water. A rocket accelerates in outer space. A rocket accelerates in outer space. If a soccer ball pushes back on your foot just as hard as your foot pushes on the ball, how can the soccer ball go anywhere? If a soccer ball pushes back on your foot just as hard as your foot pushes on the ball, how can the soccer ball go anywhere?

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Newtons 3 rd Law With Newtons 1 st and 2 nd Laws, we only talk about the forces on a single object. With Newtons 1 st and 2 nd Laws, we only talk about the forces on a single object. For N3L, we are now talking about the interaction between TWO objects. For N3L, we are now talking about the interaction between TWO objects. Symbols: Symbols: To show the force that one object exerts on another, we use a symbol like this: To show the force that one object exerts on another, we use a symbol like this: F 1-2 = the force that object 1 exerts on object 2

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Newtons 3 rd Law Example: if you kick a soccer ball, the two forces would be written like this: Example: if you kick a soccer ball, the two forces would be written like this: F f-s = the force of your foot on the soccer ball F f-s = the force of your foot on the soccer ball F s-f = the force of the soccer ball on your foot F s-f = the force of the soccer ball on your foot So, N3L says that F f-s = F s-f So, N3L says that F f-s = F s-f

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Newtons 3 rd Law The Horse-and-Cart Problem: The Horse-and-Cart Problem: Problem: If the force of the cart on the horse (F c-h ) is just as strong as the force of the horse on the cart (F h-c ), how can the horse move the cart? F h-c F c-h

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Newtons 3 rd Law Think about each object separately! Think about each object separately! First, draw a force diagram for the cart: First, draw a force diagram for the cart: Then, draw a force diagram for the horse: Then, draw a force diagram for the horse: F h-c FNFNFNFN FGFGFGFG FfFfFfFf F c-h FNFNFNFN FGFGFGFG F APPLIED FfFfFfFf

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Newtons 3 rd Law Answer: the horse can move because the force from the ground (forward) on the horse is even bigger than the force from the cart (backward)!!! Answer: the horse can move because the force from the ground (forward) on the horse is even bigger than the force from the cart (backward)!!! IMPORTANT!!! - IMPORTANT!!! - The equal and opposite forces dont cancel each other out, because they are acting on different objects!

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N3L: Force Diagrams Your assignment is to finish the N3L Force Diagrams worksheet Your assignment is to finish the N3L Force Diagrams worksheet Well do an example together. Well do an example together.

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