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**Newton’s Third Law of Motion:**

“For every action there is an equal and opposite reaction” The correct way to say it: For every action force there is an equal and opposite reaction force.

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**Forces always occur in PAIRS**

An action force… …and a reaction force

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Some examples:

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**See the pattern??? A pushes on B, so B pushes on A**

OR A pulls on B, so B pulls on A

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Question: If force pairs on two objects are always equal and opposite, how is it that sometimes only one of the objects moves????

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**For example: Shooting an arrow? Throwing a baseball?**

Hitting a hockey puck?

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Answer: Wait…… What?? The pair of forces acting on the objects has to be the same, but the REACTION of the objects to the forces does NOT have to be the same!

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**Example: a canon firing**

According to Newton’s 3rd Law of Motion, both the canon ball and the cannon experience the SAME FORCE from the gunpowder. So why does the cannon ball move so fast while the cannon only moves back a little?

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**Explanation 1 - The math:**

The Cannon: The Cannonball: BIG mass: SMALL mass So according to Newton’s 2nd Law of Motion: F M F M = A = A = SMALL acceleration! = LARGE acceleration!

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**The cannon has much more INERTIA than the cannon ball.**

Explanation 2 – Inertia: The cannon has much more INERTIA than the cannon ball. Therefore the same force can move the cannon ball much more easily than it can move the cannon!

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