Newton s 3 rd law of motion
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Newton’s 3 rd Law of Motion. Newton’s 3 rd Law. For every action there is an equal and opposite reaction. Examples. Hammer and Nail:

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Newton’s 3 rd Law of Motion

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Newton’s 3rdLaw of Motion

Newton’s 3rd Law

For every action there is an equal and opposite reaction


  • Hammer and Nail:

    • As a hammer hits a nail, the hammer applies a force to the nail AND the nail applies a force on the hammer this is why the hammer slows down as it hits the nail

  • Rocket blasting off the launch pad:

    • As the rocket applies a thrust force downward, the rocket moves upward


They are known as: “action-reaction pairs”

“Action-Reaction Pairs”

Object A applies an “action” force on Object B while at the same time Object B has a “reaction” force on Object A of equal magnitude but opposite direction.

That’s a lot to write out…

  • It can be shortened with a formula! 

  • FAB = -FBA

    • In English this simply says the Force that Object A puts on Object B is equal but opposite (remember: the negative just means opposite direction) to the Force that Object B puts on Object A

Ever try to jump with your legs straight the entire time?

Why is it so difficult?

  • There is no action-reaction pair!

  • You need the bending “action” to have the jumping “reaction”.

Think, pair, share (Think about it yourself, talk it over with a partner, and then write your ideas in your notes):

Use Newton’s 3rd Law of Motion to explain to a classmate why it hurts when you punch a wall.

Can a wall really punch you back?

YES! Well, in a way…


  • The wall applies a force that is exactly equal in magnitude but opposite in direction.

    • This “reaction” force is known as a negative force; remember the negative just refers to the fact that it is opposite in the direction it goes

Check YOUR Understanding

A truck driving down the road hits a mosquito. The mosquito is smashed and the truck continues driving.

Which of the two forces is greater: The mosquito on the truck or the truck on the mosquito? Why?

(Hint: really think about Newton’s 3rd Law here and maybe even think back to Newton’s 2nd Law to answer “why?”)

When you think you have the answer…

Use your Peer-to-Peer Network toask a classmate (that has already completed this question) if your answer matches theirs

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