# rd Newtons 3 Law of Motion rd Newtons

• Slides: 13

rd Newton’s 3 Law of Motion

rd Newton’s 3 Law For every action there is an equal and opposite reaction

Examples 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

ALL FORCES COME IN PAIRS 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 3 rd 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…

How? 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 3 rd Law here and maybe even think back to Newton’s 2 nd Law to answer “why? ”)