Transcript 12.3 Newton`s 3rd Law of Motion

Name: _______________________________ Class: ____
12.3 Newton’s 3rd Law of Motion
Chapter 12
Forces and Motion
What are we going to learn?
Newton’s 3rd Law of Motion
Newton’s 3rd Law of Motion
If one object exerts a force on another object,
then the second object exerts a force of equal
strength in the opposite direction on the first
object (for every action there is an equal but
opposite reaction)
Ex: If you leaned against a wall and it didn’t
push back on you, you’d fall through.
What does that mean?
Equal and opposite means ________________
_______________________________________
What does that mean?
Equal and opposite mean the forces have equal
strength and they act in opposite directions
Action-Reaction Pairs
Pairs of action and reaction forces are all
around you.
Ex: When you walk, you push backward on the
ground with your feet (action force) and the
ground pushed forward on your feet with an
equal and opposite force (reaction force).
Ex: A swimmer moves forward by exerting an
action force on the water with her hands. The
water pushes on her hands with an equal
reaction force that propels her body forward.
What force is not labeled?
What force is not labeled?
Gravity
Is there a force that cancels out gravity?
Is there a force that cancels out gravity?
No
What would happen to the volleyball if the
players take their hands away from it?
What would happen to the volleyball if the
players take their hands away from it?
It would fall to the ground
How does the diagram illustrate Newton’s
third law of motion?
How does the diagram illustrate Newton’s
third law of motion?
Action (foot) – Reaction (ball) forces are at work
in equal and opposite directions so the ball
moves forward
Examples
Suppose you hit a nail with a hammer, exerting
a force on the nail. What is the reaction force?
Examples
Suppose you hit a nail with a hammer, exerting
a force on the nail. What is the reaction force?
The nail exerting an opposite and equal force
on the hammer, causing the motion of the
hammer to stop
What are the action and reaction forces
involved in jumping on a trampoline?
What are the action and reaction forces
involved in jumping on a trampoline?
Action: person jumping on the trampoline
Reaction: trampoline pushing up on the person
A volunteer please...
A volunteer needs to sit on the swivel chair and
lift their feet off the floor.
The volunteer must get the chair to move
without touching the floor, the base of the chair,
or other objects.
Try it again....this time use your feet to push
against the floor.
Why is it easier to move the chair by pushing off
the floor?
_______________________________________
_______________________________________
_______________________________________
_______________________________________
Why is it easier to move the chair by pushing off
the floor?
As the person pushes against the floor, an
equal amount of force pushed the volunteer
away from the floor, causing the chair to move.
If the person cannot push against another
object, then there is no opposite reaction force
to move the chair.
Another volunteer please...
A volunteer sits on a swivel chair and tosses a
ball to another person.
What happened when the volunteer tossed the
ball?
_______________________________________
What made the volunteer move?
_______________________________________
A volunteer will sit on a swivel chair and toss a
ball to another person.
What happened when the volunteer tossed the
ball?
The volunteer moved backward
What made the volunteer move?
A force exerted by the ball on the volunteer
Detecting Motion
If you drop your pen, gravity pulls the pen
downward.
According to Newton’s 3rd law, the pen pulls
Earth upward with an equal and opposite
reaction force.
You see the pen fall, but you don’t see Earth
accelerate toward the pen.
Why???
According to Newton’s 2nd law, if mass
increases and force stays the same,
acceleration decreases.
The same force acts on both Earth and your
pen but Earth has such a large mass that its
acceleration is so small you don’t notice it.
Do Action-Reaction Forces Cancel?
If 2 equal forces acting in opposite directions on
an object cancel each other out and produce no
change in motion, why don’t the action-reaction
forces of Newton’s third law of motion cancel as
well?
Action and reaction forces do not cancel out
because they act on different objects
Ex: Swimmer exerts action on water, but water
exerts a reaction force on the swimmer’s hands
The leash pulls on the dog with a 10 N force
to the right
I get it! Now I know that Newton’s third law
states that _____________________________
I get it! Now I know that Newton’s third law
states that for every action force there is an
equal and opposite reaction force
In jet engines, the
exhaust gases are
pushed out of the
engine, and the
gases push against
the engine, pushing
the plane forward
(think of an air-filled
balloon that is opened
to release the air)
What makes the Bug go Splat?
What happened to the driver when she braked
suddenly?
What makes the Bug go Splat?
What happened to the driver when she braked
suddenly?
She felt like she was thrown forward
Why didn’t she hit the windshield when she
braked?
Why didn’t she hit the windshield when she
braked?
She was probably wearing a seatbelt
What forces act when a seatbelt stops a driver
from moving too far forward?
What forces act when a seatbelt stops a driver
from moving too far forward?
The driver’s body exerts a force on the seatbelt
(action). The seatbelt exerts a force on the
driver that keeps her body from moving forward
(reaction).
What must happen for the bug to stop moving
at constant velocity?
What must happen for the bug to stop moving
at constant velocity?
A force must act on the bug
Which has greater mass, the bug or the car?
Which has greater mass, the bug or the car?
The car has a much greater mass
How will this difference affect the collision?
How will this difference affect the collision?
The car will be much less affected by the action
force than the bug is by the reaction force
Buzz!
In order for the bug to fly through the air, a force
has to push the bug forward. Identify this force.
How does the bug produce it? (Hint: think back
to how a swimmer moves through the water.)
Buzz!
In order for the bug to fly through the air, a force
has to push the bug forward. Identify this force.
How does the bug produce it? (Hint: think back
to how a swimmer moves through the water.)
Air pushes the bug forward. The bug produces
this force by pushing backward on the air with
its wings, and the reaction force is that the air
pushes forward on the bug.
The bug was at rest on a tree when it saw the
car and decided to fly toward it. If the bug has a
mass of 0.05 kg and accelerates at 2 m/s2,
what’s the net force on the bug?
The bug was at rest on a tree when it saw the
car and decided to fly toward it. If the bug has a
mass of 0.05 kg and accelerates at 2 m/s2,
what’s the net force on the bug?
0.05 kg x 2 m/s2 = 0.1 N
Vroom!
The driver hates killing bugs. When she saw
one coming toward the windshield, she braked
suddenly and hoped it would get out of the way.
(Sadly, it did not.) When she hit the brakes, she
felt that she was thrown forward. Use one of
Newton’s laws to explain why.
Vroom!
The driver hates killing bugs. When she saw
one coming toward the windshield, she braked
suddenly and hoped it would get out of the way.
(Sadly, it did not.) When she hit the brakes, she
felt that she was thrown forward. Use one of
Newton’s laws to explain why.
Newton’s first law says that objects in motion
stay in motion. The car stopped, but she kept
moving forward.
Splat!
The unfortunate bug hits the windshield with a
force of 1 N. If you call this the action force,
what is the reaction force?
Splat!
The unfortunate bug hits the windshield with a
force of 1 N. If you call this the action force,
what is the reaction force?
The windshield
Does the car hit the bug any harder than the
bug hits the car? Use one of Newton’s laws to
explain why or why not.
Does the car hit the bug any harder than the
bug hits the car? Use one of Newton’s laws to
explain why or why not.
The windshield hits the bug with a 1 N force.
No: Newton’s third law states that for every
force, there is an equal and opposite force.
Compare the forces on the bug and the car
again. Use another one of Newton’s laws to
explain why the bug goes splat and the car
keeps going, without noticeably slowing down.
Compare the forces on the bug and the car
again. Use another one of Newton’s laws to
explain why the bug goes splat and the car
keeps going, without noticeably slowing down.
Newton’s second law: The same force acts on
both, but the bug has a much smaller mass, so
it accelerates much more
Questions
True or false. If the statement is false,
change the underlined word(s) to make the
statement true.
1. _______ If you increase the force on an
object, its acceleration increases.
Questions
True or false. If the statement is false,
change the underlined word(s) to make the
statement true.
1. TRUE If you increase the force on an
object, its acceleration increases.
2. ________ If you increase the mass of an
object, its acceleration decreases.
2. False – INCREASES If you increase the
mass of an object, its acceleration decreases.
3. ______ To accelerate a 3 kg skateboard at
9 m/s2, a force of 3 newtons is needed.
3. False - 27 N To accelerate a 3 kg
skateboard at 9 m/s2, a force of 3 newtons is
needed.
4. _________ The amount of inertia an object
has depends on its speed.
4. False – MASS The amount of inertia an
object has depends on its speed.
5. ___________ Newton’s first law of motion
states that an object will not experience a
change in motion unless acted upon by a(n)
balanced force.
5. False – UNBALANCED Newton’s first law
of motion states that an object will not
experience a change in motion unless acted
upon by a(n) balanced force.
6. ________ To increase acceleration of an
object, you reduce its mass or increase the
applied force.
6. TRUE To increase acceleration of an object,
you reduce its mass or increase the applied
force.
7. ________ Newton’s third law of motion
states that if one object exerts a force on
another object, then the second object exerts a
force of equal strength in the same direction.
7. False – OPPOSITE Newton’s third law of
motion states that if one object exerts a force
on another object, then the second object
exerts a force of equal strength in the same
direction.
8. _________ Resistance to change in motion
is called stasis.
8. False – INERTIA Resistance to change in
motion is called stasis.
9. ________ Action and reaction forces acting
in opposite directions do cancel out because
they act on different objects.
9. False - DO NOT Action and reaction forces
acting in opposite directions do cancel out
because they act on different objects.
10. ________ If you lean against a wall, the
wall pushes back on you with a(n) weaker
force.
10. False – EQUAL If you lean against a wall,
the wall pushes back on you with a(n) weaker
force.
11. ________ Newton’s second law of motion
states that an object’s acceleration depends on
its distance and on the net force acting on it.
11. False – Mass Newton’s second law of
motion states that an object’s acceleration
depends on its distance and on the net force
acting on it.
12. _________ Acceleration is measured in
m/s2.
12. True Acceleration is measured in m/s2.
13. _______ Force is measured in a unit called
the N.
13. True Force is measured in a unit called the
N.
14. ________ The smaller the mass of an
object, the more its inertia.
14. False – less The smaller the mass of an
object, the more its inertia.