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Warm-up: Identify the action force and the corresponding reaction force for each event: A fish swims through water. 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?

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Newton’s 3rd Law With Newton’s 1st and 2nd Laws, we only talk about the forces on a single object. For N3L, we are now talking about the interaction between TWO objects. Symbols: To show the force that one object exerts on another, we use a symbol like this: F1-2 = the force that object 1 exerts on object 2

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

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**Newton’s 3rd Law The Horse-and-Cart Problem:**

Problem: If the force of the cart on the horse (Fc-h) is just as strong as the force of the horse on the cart (Fh-c), how can the horse move the cart? Fh-c Fc-h

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**Newton’s 3rd Law Think about each object separately! Fh-c Fc-h**

First, draw a force diagram for the cart: Then, draw a force diagram for the horse: FN Fh-c Ff FG FN Fc-h Ff FAPPLIED FG

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Newton’s 3rd 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)!!! IMPORTANT!!! - The “equal and opposite” forces don’t 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 We’ll do an example together.

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