Transcript Newton`s 3rd Law

Newton’s
rd
3
Law
For every action…..
Newton's Third Law
• “For every action there is
an equal and opposite
reaction.”
• Newton’s third law
discusses pairs of objects
and the interactions
between them.
• The normal force is an
example we’ve already
seen.
Newton’s Third Law
• Another (and probably better) way of
saying this is:
For every force, there is an equal but
opposite reaction force.
• 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.
You might be wondering…
• If the wall is pushing on me, then why don’t I
move?
Another Example:
• When you throw a ball, you exert a force on the
ball. That part is obvious.
• It may not feel like the ball is exerting an equally
strong force on you.
• But what if you were standing on a skateboard,
and then you threw a ball as hard as you could?
What would happen?
Newton’s Third Law
Action and Reaction
Forces Don’t Cancel
• The forces exerted by two objects on each
other are often called an action-reaction
force pair.
• 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.
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.
Newton’s Third Law
• When a bird flies, its wings push in a
downward and a backward direction.
• 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.
Newton’s Third Law
• When you walk
forward, you
push backward
on the ground.
• Your shoe pushes
Earth backward,
and Earth pushes
your shoe forward.
3rd Law Questions
• Identify the action reaction pair for a man
paddling a boat.
– A. Man pushes on paddle, boat pushes on man.
– B. Paddle pushes on water, water pushes on
paddle.
– C. Paddle pushes on water, water pushes on man.
– D. Man pushes on paddle, water pushes on boat.
3rd Law Questions
• Choose the answer that only involves two
objects and follows the idea of :
A acts on B – B acts on A
Falling objects
• What force is being exerted that makes an
object falls to the ground?
• What is the reaction force?
Think of a falling apple….
• The earth exerts a downward force on the
apple (gravity), so the apple exerts an equal
upward force on the earth.
Large and Small Objects
• If the apple is exerting an upward force on
the Earth, why doesn’t the earth move
upward?
• Earth has so much mass compared to the
apple that it does not move noticeably when
the apple pulls on it.
The forces are the same…
Force on apple = Force on earth
However,
aapple =
aearth =
Newton's Third Law
• The astronauts on a space walk have a problem when they need
to move around in space: There is nothing to push on.
• The solution is to throw something opposite the direction they
want to move.
• This works because all forces always come in pairs.
Newton’s Third Law
A Rocket Launch
• When the rocket fuel is ignited, a hot gas
is produced.
• 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 by the rocket on the
gas molecules is the action force.
• The reaction force is the upward push on
the rocket engine by the gas molecules.
• This is the thrust that propels the rocket
upward.
Newton's Third Law
• Allows us to move!
Check your understanding….
• While driving down the road, a firefly strikes
the windshield of a bus and makes a quite
obvious mess in front of the face of the driver.
This is a clear case of Newton's third law of
motion. The firefly hit the bus and the bus hits
the firefly. Which of the two forces is greater:
the force on the firefly or the force on the
bus?
Check your understanding…
• Many people are familiar with the fact that a rifle
recoils when fired. A gunpowder explosion creates
hot gases that expand outward allowing the rifle to
push forward on the bullet. Consistent with
Newton's third law of motion, the bullet pushes
backwards upon the rifle. The acceleration of the
recoiling rifle is ...
a. greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
c. the same size as the acceleration of the bullet.
Practice Problems
If an 50 Kg man jumps off a 200 kg boat with force of 100 N,
what is the acceleration of the boat?
An 80 kg astronaut throws a 10 kg air tank with an acceleration
of 3 m/s. What’s the astronaut’s acceleration?