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Newton’s Third Law of Motion Action and Reaction

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Questions I should be able to answer when we are done What is Newton’s 3 rd law? What are force pairs? Force pairs are always ______ and ______? When do force pairs cancel each other out? When do they not? What do force, mass, and acceleration have to do with Newton’s 3 rd law?

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Question: Is it possible for me to touch the table but not have the table touching me?

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Forces and Interactions A force is a push or pull. Forces do not act alone. Forces are mutual actions called INTERACTIONS between two things.

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For example… A hammer hits a nail… The nail interacts with the hammer. The hammer hits the nail and the nail hits the hammer.

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Newton’s Third Law “Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first”

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In other words… “For every action there is an EQUAL and OPPOSITE reaction”

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Action vs. Reaction For example: One Force (an action): Pushing on a wall Second Force (a reaction): The wall pushing back

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What happens to you when you jump off a boat onto the shore? What happens to the boat?

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Action and Reaction “ACTION” happens first: Driving in a car, the tires push on the road. Dropping a book, the Earth pulls on the book. “REACTION” happens second: The road pushes back on the tires. The book pushes back on the Earth.

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Action and Reaction on Different Masses FORCES are equal even if MASSES are different.

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Action and Reaction on Different Masses The force exerted on a cannon is equal to the force exerted on the cannonball So, why does the cannonball move so much faster and farther than the cannon?

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Why don’t they cancel each other out? Do the action and reaction act on the same object? If so, they can cancel each other out.

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Action VS Reaction Equal in magnitude and opposite in direction Forces do not cancel each other out.

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The Horse-Cart Problem Observe the pairs of forces acting on the horse and cart

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ACTION = REACTION Example: Pushing on a wall vs Punching a wall

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Vocabulary 1. action force3. Newton’s 3 rd Law 2. interaction4. reaction force

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Questions I should be able to answer when we are done What is Newton’s 3 rd law? What are force pairs? Force pairs are always ______ and ______? When do force pairs cancel each other out? When do they not? What do force, mass, and acceleration have to do with Newton’s 3 rd law?

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