Download Newton`s third Law

NEWTON’S THIRD LAW
Equal & Opposite Forces
rd
3
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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
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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?
ACTION- Earth’s
Gravity pulls person
down
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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!
REACTIONPerson’s gravity
pulls Earth up
An object pulls upward on the Earth as much as the
Earth pulls it downward
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Object’s mass is much less than Earths, so object falls down
rather than Earth rising up to object
Draw all forces on the car/Earth system
as it is driving down the road
The road
pushing the
car forward
The car’s
tire
pushing
on the
road
Normal
force of
the Earth
pushing
up on the
car
Contact force
of the car
pushing down
on the Earth
The road’s
friction on
the car’s
tires
**Action/Reaction pairs
have matching colors in
drawing
Earth’s
Gravity
pulling the
car down
The cars
friction on
the road
The car’s
gravity
pulling the
Earth up
Elevators & Normal Force
Elevator Acceleration Down
Elevator at rest
FN=900 N
FN=890 N
W=900 N
FN=910 N
W=900 N
FNET=-10 N
Elevator accelerating Up
W=900 N
FNET=0 N
FNET=+10 N
Normal Force
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“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?
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Car??
Rocket??
Person??
Examples
Swimming, rowing
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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??
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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
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Cannon and cannonball
Do both the cannon and the cannonball experience
the same force the cannon is fired?
 Yes,
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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??
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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
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