Newton’s Third Law of

Motion

Chapter 3Section 3

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.

Action & Reaction

• What is the action and reaction of the diver?

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.