Transcript What is Newton`s Third Law

What is Newton’s Third Law
• Describes what
happens when
one object exerts
a force on
another object.
• Forces always
act in equal but
opposite pairs.
Review: What is Newton’s First
• Question:
–What is Newton’s
First Law?
Review: What is Newton’s First
• If the forces acting on
the object are
balanced, the object
will remain at rest or
stay in motion at a
constant velocity.
• If the forces are
unbalanced, the
object will accelerate
in the direction of the
net force.
Review: What is Newton’s Second
•Question:
–What is Newton’s
Second Law of
Motion?
Review: What is Newton’s Second
• An object acted upon
by a force will
accelerate in the
direction of the force.
a = f/m
f = ma
What is Newton’s Third Law
• Question:
–If you push
on a wall
does the wall
also push on
you?
What is Newton’s Third Law
• The wall actually also
pushes on you.
• So when you
push on a wall
the wall pushes
back on you with
a force equal in
strength to the
force you
exerted.
What is Newton’s Third Law
• Forces always act in equal but
opposite pairs.
• Another way to say this is:
–For every action, there is
an equal but opposite
reaction.
What is Newton’s Third Law
• When one object
exerts a force on
another object,
the second
object exerts the
same size force
on the first
object.
What are Action and Reaction
Forces?
• Forces exerted
by two objects
on each other
are often called
an actionreaction pair.
What are Action and Reaction
Forces?
• Question:
–Since the
action-reaction
forces are
equal and
opposite do
they cancel?
What are Action and Reaction
Forces?
• Action-reaction
forces don’t
cancel because
they act on
different objects.
Forces can only
cancel if they act
on the same
object.
What are Action and Reaction
Forces?
• Here are 5 examples of action and reaction forces:
1.When you push on a wall the wall pushes back on you.
2. When you walk on the the ground, you push the
ground and the ground pushes you.
3.When the bat hits the ball the ball hits the bat.
4.When your finger touches your nose your nose
touches your finger.
5. When you push on a table the table pushes on you.
What are Action and Reaction
Forces?
• Newton’s 3rd law applies to driving
because when you drive the action force is
the pushing against the road and the
reaction is the road pushing against the
tires. Also when a car hits a person, the
person hits the car. When a car hits
telephone pole the pole hits the car. When
a car hits a wall the wall hits the car.
What are Action and Reaction
Forces?
• Question:
– Explain using
Newton’s Third
Law why is
driving on icy
roads so
difficult.
What are Action and Reaction
Forces?
• It is difficult to drive on a icy road
because the tire can not grip the road
because there is less fiction. So when
the car tries to push on the road it can
not because the road is icy. If the car
can't push on the road, the road can't
push on the car. As a result the car
slides in the ice
What are Action and Reaction
Forces?
• You constantly use
action-reaction forces
as you move about.
– When you jump,
you push on the
ground. The
ground pushes up
on you. It is this
upward force that
pushes you into the
air.
What are Action and Reaction
Forces?
• When a bird flies, its wings push in a downward and
backward direction. This pushes air downward and
backward.
– By Newton’s third, the air pushes back on the
bird in the opposite direction; upward and
forward.
What are Action and Reaction
Forces?
• Question:
–Explain if
the photo
shows an
actionreaction.
What are Action and Reaction
Forces?
• Your shoe
pushes Earth
backward, and
Earth pushes
your shoe
forward.
What are Action and Reaction
Forces?
• Some action-reaction pairs are not
noticeable because one of the objects
is much more massive and appears
to remain motionless when a force
acts upon it.
• Question:
–Give an example of the above.
What are Action and Reaction
Forces?
• Earth has so much
mass compared to
you that it does not
move noticeably
when you push it. If
you step on
something with less
mass than you do,
you can see it being
pushed back.
What are Action and Reaction
Forces?
• Question:
–How is a
rocket launch
an example
of Newton’s
Third Law?
What are Action and Reaction
Forces?
• Rocket engines
supply the force,
called thrust, that
lifts the rocket.
When the rocket
fuel is ignited, a
hot gas is
produced.
What are Action and Reaction
Forces?
• As gas molecules
collide with the
inside walls, the
walls exert a force
that pushes them
out of the bottom of
the engine. This
downward push is
the action force.
What are Action and Reaction
Forces?
• The reaction
force is the
upward push on
the rocket
engine by the
gas molecules.
This is the thrust
that propels the
rocket upward.
How do You Measure Weight?
• When you stand
on a scale, your
weight pushes
down on the
scale. This
causes the scale
pointer to point
to your weight.
How do You Measure Weight?
• At the same time,
by Newton’s Third
Law the scale
pushes up on you
with a force equal
to your weight. This
force balances the
downward pull of
gravity on you.
Free Fall and Weightlessness
• Imagine standing on a
scale in an elevator
that is falling. Inside
the elevator you and
the scale are both in a
free fall. The only
force acting upon you
is gravity, the scale is
no longer pushing up
on you.
Free Fall and Weightlessness
• According to Newton’s Third
Law, you no longer push down
on the scale. So the scale
pointer stays at zero and you
seem to be weightless.
• Weightlessness is the
condition that occurs in free
fall when the weight of an
object seems to be zero.
Weightlessness in Orbit
• Imagine you were
holding a ball in the
free-falling elevator. If
you let the ball go, the
position of the ball
relative to you and the
elevator would not
change, because
everything is moving
at the same speed.
Weightlessness in Orbit
• Using the same ball,
suppose you push the ball
downward. This pushing
force adds to the
downward force of gravity.
Now the acceleration of
the ball is greater than the
acceleration of you and
the elevator. The ball will
continue to move faster
downward until it hits the
elevator floor.
Weightlessness in Orbit
• When the space shuttle orbits the Earth, the
shuttle and all objects in it are in a free fall.
Weightlessness in Orbit
• The shuttle and all
object within are
falling in a curved
path around the
Earth, instead of
falling straight
downward. As a result
the objects in the
shuttle appear to be
weightless.