Law of interaction

Newton's third law of motion

1. The Law of InteractionThe Law of Interaction
Newton’s Third Law of MotionNewton’s Third Law of Motion

2. 33
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Law Law

For every action force, there is For every action force, there is
an equal and opposite reaction an equal and opposite reaction
force.force.
In equation: FIn equation: F
A on B A on B = -F= -F
B on A B on A

3. 
Forces occur in pairs.Forces occur in pairs.

Example: for object A and B:Example: for object A and B:
Action force: “A pushes B”Action force: “A pushes B”
Reaction force: “B pushes A”Reaction force: “B pushes A”

The action and reaction forces occur at The action and reaction forces occur at
exactly the same time!exactly the same time!

If A pushes B, then B at the same time If A pushes B, then B at the same time
pushes A pushes A EVEN IF B HAS NO LIFE!EVEN IF B HAS NO LIFE!

4. 
““Equal”Equal”
In Newton’s Third Law, “equal” means:In Newton’s Third Law, “equal” means:
Equal in magnitude (strength)Equal in magnitude (strength)

The action and reaction forces have The action and reaction forces have
exactly the same MAGNITUDE.exactly the same MAGNITUDE.
Example: If Bob pushes the wall with 400 Example: If Bob pushes the wall with 400
N of force, then the wall pushes back Bob N of force, then the wall pushes back Bob
with 400 N of force too.with 400 N of force too.

5. 
““Opposite”Opposite”
In Newton’s Third Law, “opposite” means:In Newton’s Third Law, “opposite” means:
Opposite in direction.Opposite in direction.

Example: Example:
If Bob’s foot kicks the soccer ball forward, If Bob’s foot kicks the soccer ball forward,
then the soccer ball kicks Bob’s foot then the soccer ball kicks Bob’s foot
backwards.backwards.

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Law Law
According to Newton, According to Newton,
whenever objects A and whenever objects A and
B interact with each B interact with each
other, they exert forces other, they exert forces
upon each other. When upon each other. When
you sit in your chair, you sit in your chair,
your body exerts a your body exerts a
downward force on the downward force on the
chair and the chair chair and the chair
exerts an upward force exerts an upward force
on your body. on your body.

7. 33
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Law Law
There are two forces There are two forces
resulting from this resulting from this
interaction - a force interaction - a force
on the chair and a on the chair and a
force on your body. force on your body.
These two forces are These two forces are
called called actionaction and and
reactionreaction forces. forces.

8. Action: earth pulls on you
Reaction: you pull on earth
Action and Reaction on Different Masses
Consider you and the earth

9. Newton’s 3rd Law in NatureNewton’s 3rd Law in Nature

Consider the propulsion of a Consider the propulsion of a
fish through the water. A fish through the water. A
fish uses its fins to push fish uses its fins to push
water backwards. In turn, water backwards. In turn,
the water the water reactsreacts by pushing by pushing
the fish forwards, propelling the fish forwards, propelling
the fish through the water.the fish through the water.

The size of the force on the The size of the force on the
water equals the size of the water equals the size of the
force on the fish; the force on the fish; the
direction of the force on the direction of the force on the
water (backwards) is water (backwards) is
opposite the direction of the opposite the direction of the
force on the fish (forwards).force on the fish (forwards).

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Law Law
Flying gracefully Flying gracefully
through the air, birds through the air, birds
depend on Newton’s depend on Newton’s
third law of motion. As third law of motion. As
the birds push down on the birds push down on
the air with their wings, the air with their wings,
the air pushes their the air pushes their
wings up and gives wings up and gives
them lift.them lift.

11. 
Consider the flying motion of birds. A bird flies by Consider the flying motion of birds. A bird flies by
use of its wings. The wings of a bird push air use of its wings. The wings of a bird push air
downwards. In turn, the air reacts by pushing the bird downwards. In turn, the air reacts by pushing the bird
upwards. upwards.

The size of the force on the air equals the size of the The size of the force on the air equals the size of the
force on the bird; the direction of the force on the air force on the bird; the direction of the force on the air
(downwards) is opposite the direction of the force on (downwards) is opposite the direction of the force on
the bird (upwards).the bird (upwards).

Action-reaction force pairs make it possible for birds Action-reaction force pairs make it possible for birds
to fly.to fly.

13. Other examples of Newton’s Other examples of Newton’s
Third LawThird Law

The baseball forces The baseball forces
the bat to the left (an the bat to the left (an
action); the bat action); the bat
forces the ball to the forces the ball to the
right (the reaction). right (the reaction).

14. 33
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Law Law

Consider the motion of Consider the motion of
a car on the way to a car on the way to
school. A car is school. A car is
equipped with wheels equipped with wheels
which spin backwards. which spin backwards.
As the wheels spin As the wheels spin
backwards, they grip the backwards, they grip the
road and push the road road and push the road
backwards.backwards.

15. Action: tire pushes on road
Reaction: road pushes on tire

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Law Law
The reaction of a rocket is The reaction of a rocket is
an application of the third an application of the third
law of motion. Various law of motion. Various
fuels are burned in the fuels are burned in the
engine, producing hot engine, producing hot
gases. gases.
The hot gases push against The hot gases push against
the inside tube of the rocket the inside tube of the rocket
and escape out the bottom and escape out the bottom
of the tube. As the gases of the tube. As the gases
move downward, the rocket move downward, the rocket
moves in the opposite moves in the opposite
direction.direction.

17. Action: rocket pushes on gases
Reaction: gases push on rocket

18. Newton’s 3Newton’s 3
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Law Law

Suppose you are taking a space walk Suppose you are taking a space walk
near the space shuttle, and your near the space shuttle, and your
safety line breaks. How would you safety line breaks. How would you
get back to the shuttle?get back to the shuttle?

19. Newton’s 3Newton’s 3
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Law Law

The thing to do would be to take one of the tools from The thing to do would be to take one of the tools from
your tool belt and throw it as hard as you can directly your tool belt and throw it as hard as you can directly
away from the shuttle. Then, with the help of away from the shuttle. Then, with the help of
Newton's second and third laws, you will accelerate Newton's second and third laws, you will accelerate
back towards the shuttle. As you throw the tool, you back towards the shuttle. As you throw the tool, you
push against it, causing it to accelerate. At the same push against it, causing it to accelerate. At the same
time, by Newton's third law, the tool is pushing back time, by Newton's third law, the tool is pushing back
against you in the opposite direction, which causes against you in the opposite direction, which causes
you to accelerate back towards the shuttle, as desired. you to accelerate back towards the shuttle, as desired.

20. ReviewReview
Newton’s First Law:
Objects in motion tend to stay in motion
and objects at rest tend to stay at rest
unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration
(F = ma).
Newton’s Third Law:
For every action there is an equal and
opposite reaction.

21. 1
st
law: Homer is large and
has much mass, therefore he
has much inertia. Friction
and gravity oppose his
motion.
2
nd
law: Homer’s mass x
9.8 m/s/s equals his
weight, which is a force.
3
rd
law: Homer pushes
against the ground and it
pushes back.