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Newton’s third Law

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Newton’s third Law

Equal & Opposite Forces

- 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 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

ACTION- Earth’s Gravity pulls person down

- 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

REACTION-Person’s gravity pulls Earth up

The road’s friction on the car’s tires

**Action/Reaction pairs have matching colors in drawing

Normal force of the Earth pushing up on the car

Earth’s Gravity pulling the car down

The road pushing the car forward

The car’s tire pushing on the road

The car’s gravity pulling the Earth up

Contact force of the car pushing down on the Earth

The cars friction on the road

Elevator Acceleration Down

Elevator at rest

Elevator accelerating Up

FN=900 N

FN=890 N

FN=910 N

W=900 N

W=900 N

W=900 N

FNET=+10 N

FNET=-10 N

FNET=0 N

- “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”

- Car??
- Rocket??
- Person??

- Swimmer pushes backward on the water…… the water pushes swimmer forward
- The oar pushes water backward… water pushes oar and boat forward

- 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!!

- 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

- 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

- No, the action force,