NEWTON’S THIRD LAW

Equal & Opposite Forces

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

Elevators & Normal Force

Elevator Acceleration Down

Elevator at rest

Elevator accelerating Up

W=900 N

FN=890 N FN=900 N

W=900 NW=900 N

FN=910 N

FNET=+10 N

FNET=0 NFNET=-10 N

Normal Force

“Normal” means perpendicular Force of the Earth (ground) pushing up on an

object when they are in contact with each other

If object is on a horizontal surface and no other vertical forces are applied, then Normal Force is Equal to the weight (force of gravity) of the object

Normal Force is NOT the reaction to the force of gravity acting on an object

Responsible for how heavy we “feel” AKA “Apparent Weight”

Astronauts & Reaction Forces

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