newton’s third law
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Newton’s third Law. Equal & Opposite Forces. 3 rd 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” - PowerPoint PPT PresentationTRANSCRIPT
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