newton’s 3rd law - mr. finke's science class · newton’s third law •this pushes air...
TRANSCRIPT
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