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Newton’s Third Law of Motion
• If you pushed against a wall, what would happen?
• Nothing• What would happen if you
had roller skates on? • You would roll backward.• The harder you push? The
farther you roll
Newton’s Third Law of Motion
• Your action caused a reaction.• Your pushing on the wall caused you
to roll backward.• Newton’s Third Law of Motion states:• When an object exerts a force on a
second object, the second one exerts a force on the first that is equal and opposite in direction.
• AKA – For every action there is an equal and opposite reaction.
Action & Reaction
• When you apply a force on something a reaction occurs.
• When you jump on a trampoline, you exert a downward force on the trampoline.
• The trampoline then exerts an equal upward force on you, sending you into the air.
Action & Reaction
• Remember that action-reaction forces are acting on different objects.
• Even though forces are equal, they are not balanced because they act on different objects.
Action & Reaction
• What is the action and reaction of a swimmer pushing off the wall?
• As the swimmer pushes on the wall, the wall exerts a force pushing her forward.
• As the swimmer pushes the water out of the way, the water pushes the swimmer back.
Rocket Propulsion
• In a rocket engine, burning fuel makes a hot gas.
• The engine exerts a force on the gases causing the gases to escape outside the back of the rocket.
• The gases exert a force on the rocket and push it forward.
Momentum
• Which is easier to stop?• A slow moving bike or a fast moving
bike?• A slow moving bike or a slow moving
car?
Momentum
• Increasing the speed or mass of an object makes it harder to stop.
• Momentum is the product of an objects mass and velocity.
• Momentum is abbreviated with the symbol p.
• p = mv• Momentum = (mass)(velocity)• Momentum = kg m/s
The Law of Conservation of Momentum
• The momentum of an object doesn’t change unless its mass, velocity, or both change.
• Momentum can be transferred from one object to another.
The Law of Conservation of
Momentum• What happens when a bowling ball hits the
pins? If there is no other forces applied…• The total momentum of the bowling ball is
transferred to the pins and the pins go flying.
• The momentum that the pins gained is equal to the total momentum the bowling ball had before it hit the pins.
• Momentum is not lost or created only transferred.
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