Read pp. 150-153 Name 3 action/reaction force pairs. How does playing pool represent both Newton’s 3rd Law and the conservation of momentum? Food for thought: When an object puts a force on another object, that object pushes back with equal force. How can objects still move?

Newton’s 3rd Law of Motion

Newton’s 3rd Law For every ACTION there is an equal and opposite REACTION.

Think about it . . . What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force. Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

Newton’s Third Law A bug with a mass of 5 grams flies into the windshield of a moving 1000 kg bus. Which will have the most force? The bug on the bus The bus on the bug

Newton’s Third Law The force on the bug would be THE SAME as the force on the bus. Force (bug)= m x A Force (bus)= M x a Think I look bad? You should see the other guy!

A few examples…

Action: tire pushes on road Reaction: road pushes on tire Ask students: Have you ever noticed that the wheels on a car spin the opposite direction as the car’s movement? What’s up with that? Isn’t it the tires that move your car? (No – it’s the push of the road back on the tires that move it) Action: tire pushes on road Reaction: road pushes on tire

Reaction: gases push on rocket Action: rocket pushes on gases

Important Vocab Action force: initial force applied on something Reaction force: response (equal and opposite) to the action force

Consider hitting a baseball with a bat Consider hitting a baseball with a bat. If we call the force of the bat on the ball the action force, identify the reaction force. (a) the force of your hands on the bat (b) the force of the ball on the bat (c) the force the ball carries with it in flight (d) the force of your arms during the swing

Consider hitting a baseball with a bat Consider hitting a baseball with a bat. If we call the force of the bat on the ball the action force, identify the reaction force. (a) the force of your hands on the bat (b) the force of the ball on the bat (c) the force the ball carries with it in flight (d) the force of your arms during the swing

Identify the action/reaction forces in each example below. 1. Push down on your desk: Action: Reaction: 2. Kicking a soccer ball 3. Jumping up off the ground Reaction:

A bright student once asked me… If action/reaction pairs are always equal (balanced forces), then how do objects ever move? (unbalanced forces)

Newton’s Cart From LaFart What are the action/reaction pairs? Why does the cart move??? Newton’s Cart From LaFart 100 N Big Bad Billy Little Leroy 10 N

http://www.youtube.com/watch?v=8bTdMmNZm2M How is Newton’s 3rd Law illustrated in the video? The force of the Earth on the moon keeps the moon in orbit. Why isn’t the Earth affected by the moon’s force?

Newton’s 3rd Law: Apply It!

Newton’s 3rd Law: Apply It!

Newton’s Cart From LaFart Give an everyday example of Newton’s 3rd Law. Newton’s Cart From LaFart 300 N Big Bad Billy Little Leroy 50 N Identify the Action/Reaction Pairs in the diagram above. Action 1: the force of ______________ on ______________ Reaction 1: the force of ______________ on ______________ Action 2: the force of ______________ on ______________ Reaction 2: the force of ______________ on ______________ What is the NET FORCE on THE CART? ____ N to the ______________ Therefore, even though the action/reaction pairs are equal and opposite the net force on the cart is BALANCED / UNBALANCED so the cart ______________ to the ______________. To determine NET FORCE, tell students to think about the ACTION FORCES separately from the REACTION FORCES because they have different effects. In this case the effect of the action force is the movement of the cart. The effect of the reaction force is that the cart can absorb the force and your hand feels it “push back”.