Agenda

• Island Hopping

• BrainBashers

• Review Newton’s Laws: Inertia, F=MA

• Newton’s 3rd Law: Equal and Opposite

Action(Action-Reaction)

BrainBashers

• Dribble

Dribble

BrainBashers

• knee

light light

BrainBashers

• Beating

Beating Bush Beating

Beating

Newton’s 3rd Law

For every action…..

Action and Reaction

• Newton’s third law describes something else that happens when one object exerts a force on another object.

3

Newton’s Third Law

• According to Newton’s third law of motion, forces always act in equal but opposite pairs.

Action and Reaction

• Another way of saying this is for every action, there is an equal but opposite reaction.

3

Newton’s Third Law

• This means that when you push on a wall, the wall pushes back on you with a force equal in strength to the force you exerted.

Action and Reaction Forces Don’t Cancel

• The forces exerted by two objects on each other are often called and action-reaction force pair.

3

Newton’s Third Law

• Either force can be considered the action force or the reaction force.

• Action and reaction force pairs don’t cancel because they act on different objects.

• You constantly use action-reaction force pairs as you move about.

3

Newton’s Third Law

• When you jump, you push down on the ground.

• The ground then pushes up on you. It is this upward force that pushes you into the air.

Action and Reaction Forces Don’t Cancel

Action and Reaction Forces Don’t Cancel

• When a bird flies, its wings push in a downward and a backward direction.

3

Newton’s Third Law

• This pushes air downward and backward.

• By Newton’s third law, the air pushes back on the bird in the opposite directions—upward and forward.

• This force keeps a bird in the air and propels it forward.

Large and Small Objects

• When you walk forward, you push backward on the ground.

3

Newton’s Third Law

• Your shoe pushes Earth backward, and Earth pushes your shoe forward.

Large and Small Objects

• Earth has so much mass compared to you that it does not move noticeably when you push it.

3

Newton’s Third Law

• If you step on something that has less mass than you do, like a skateboard, you can see it being pushed back.

A Rocket Launch

• When the rocket fuel is ignited, a hot gas is produced.

3

Newton’s Third Law

• As the gas molecules collide with the inside engine walls, the walls exert a force that pushes them out of the bottom of the engine.

A Rocket Launch

• This downward push is the action force.

3

Newton’s Third Law

• The reaction force is the upward push on the rocket engine by the gas molecules.

• This is the thrust that propels the rocket upward.

Weightlessness • You might have seen pictures of astronauts

floating inside a space shuttle as it orbits Earth.

3

Newton’s Third Law

• The astronauts are said to be weightless.

• Yet the force of gravity on the shuttle is almost 90 percent as large as at Earth’s surface.

• Newton’s laws of motion can explain why the astronauts float as if there were no forces acting on them.

Measuring Weight

• When you stand on a scale, your weight pushes down on the scale.

3

Newton’s Third Law

• This causes the scale pointer to point to your weight.

Measuring Weight

• At the same time, by Newton’s third law the scale pushes up on you with a force equal to your weight.

3

Newton’s Third Law

• This fore balances the downward pull of gravity on you.

Free Fall and Weightlessness

• Now suppose you were standing on a scale in an elevator that is falling.

3

Newton’s Third Law

• A falling object is in free fall when the only force acting on the force is gravity.

• You and the scale are both in free fall.

Free Fall and Weightlessness

• Because the only force acting on you is gravity, the scale no longer is pushing up on you.

3

Newton’s Third Law

• According to Newton’s third law, you no longer push down on the scale.

Free Fall and Weightlessness

• So the scale pointer stays at zero and you seem to be weightless.

3

Newton’s Third Law

• Weightlessness is the condition that occurs in free fall when the weight of an object seems to be zero.

3rd

Law

• Forces are not a thing of themselves, but

are part of a mutual interaction

• Forces only exist in pairs

• Third Law says….

– “For every force there is exist another force

that is equal in magnitude but opposite in

direction”

– You cannot push on something without it

pushing on you by the same amount in the

opposite direction

Action & Reaction

• Action forces produce the interaction, and

the reaction force must be in existence at

the same time, have the same size, and

be opposite in direction

• You apply a 10 N eastward force to wall,

wall applies a 10 N westward force to you

What is the reaction to the Force

of Gravity?

Earth’s gravity pulls objects towards it

So if all forces have are part of a pair, what is the reaction force to this??

Object’s gravity pulling Earth up!

An object pulls upward on the Earth as much as the Earth pulls it downward Object’s mass is much less than Earths, so object

falls down rather than Earth rising up to object

ACTION- Earth’s

Gravity pulls

person down

REACTION-

Person’s

gravity pulls

Earth up

Draw all forces on the car/Earth

system as it is driving down the

road Normal

force of

the

Earth

pushing

up on

the car

Contact force

of the car

pushing

down on the

Earth

Earth’s

Gravity

pulling the

car down

The car’s

gravity

pulling the

Earth up

The

car’s

tire

pushing

on the

road

The road

pushing

the car

forward

The

road’s

friction on

the car’s

tires

The cars

friction

on the

road

**Action/Reaction pairs

have matching colors in

drawing

How do objects move? What

pushes them?

• Car??

• Rocket??

• Person??

Examples

Swimming, rowing

• Swimmer pushes backward on the

water…… the water pushes swimmer

forward

• The oar pushes water backward… water

pushes oar and boat forward

Stuck in Space??

• An astronaut in space…not moving with just a

spacesuit on and he’s not tethered to the ship.

If he is only a few meters away from his ship

and there are no other people to help him how

could you get back to his spaceship? Oh No!!

Forces on different masses

• Cannon and cannonball

• Do both the cannon and the cannonball

experience the same force the cannon is

fired?

– Yes, the force on each are the

Action/Reaction pairs

• Does the cannon or the cannon ball have

a larger acceleration?

– The ball, b/c it has a small mass. It is easier

to accel. a small mass

– a = F/m

Do the forces cancel??

If every force produces a force that is equal and opposite, why don’t all the forces cancel out and always just have a net force = 0??

If the apple pulls on the cart, and the cart pulls back on the apple by the same amount in the opposite directions, wouldn’t the two forces just cancel each other? No, the action force,

And the reaction force

Are acting on diff.

Objects and therefore will not cancel