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. 33rdrd
LawLaw
 For every action force, there isFor every action force, there is
an equal and opposite reactionan equal and opposite reaction
force.force.
 In equation: FIn equation: FAonBAonB = -F= -FBonABonA

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 atThe action and reaction forces occur at
exactly the same time!exactly the same time!
 If A pushes B, then B at the same timeIf A pushes B, then B at the same time
pushes Apushes 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 haveThe action and reaction forces have
exactly the same MAGNITUDE.exactly the same MAGNITUDE.
Example: If Bob pushes the wall with 400Example: If Bob pushes the wall with 400
N of force, then the wall pushes back BobN 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 footthen the soccer ball kicks Bob’s foot
backwards.backwards.

6. 33rdrd
LawLaw
According to Newton,According to Newton,
whenever objects A andwhenever objects A and
B interact with eachB interact with each
other, they exert forcesother, they exert forces
upon each other. Whenupon each other. When
you sit in your chair,you sit in your chair,
your body exerts ayour body exerts a
downward force on thedownward force on the
chair and the chairchair and the chair
exerts an upward forceexerts an upward force
on your body.on your body.

7. 33rdrd
LawLaw
There are two forcesThere are two forces
resulting from thisresulting from this
interaction - a forceinteraction - a force
on the chair and aon the chair and a
force on your body.force on your body.
These two forces areThese two forces are
calledcalled actionaction andand
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 aConsider the propulsion of a
fish through the water. Afish through the water. A
fish uses its fins to pushfish uses its fins to push
water backwards. In turn,water backwards. In turn,
the waterthe water reactsreacts by pushingby pushing
the fish forwards, propellingthe fish forwards, propelling
the fish through the water.the fish through the water.
 The size of the force on theThe size of the force on the
water equals the size of thewater equals the size of the
force on the fish; theforce on the fish; the
direction of the force on thedirection of the force on the
water (backwards) iswater (backwards) is
opposite the direction of theopposite the direction of the
force on the fish (forwards).force on the fish (forwards).

10. 33rdrd
LawLaw
Flying gracefullyFlying gracefully
through the air, birdsthrough the air, birds
depend on Newton’sdepend on Newton’s
third law of motion. Asthird law of motion. As
the birds push down onthe birds push down on
the air with their wings,the air with their wings,
the air pushes theirthe air pushes their
wings up and giveswings up and gives
them lift.them lift.

11.  Consider the flying motion of birds. A bird flies byConsider the flying motion of birds. A bird flies by
use of its wings. The wings of a bird push airuse of its wings. The wings of a bird push air
downwards. In turn, the air reacts by pushing the birddownwards. In turn, the air reacts by pushing the bird
upwards.upwards.
 The size of the force on the air equals the size of theThe size of the force on the air equals the size of the
force on the bird; the direction of the force on the airforce 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 birdsAction-reaction force pairs make it possible for birds
to fly.to fly.

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

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LawLaw
 Consider the motion ofConsider the motion of
a car on the way toa car on the way to
school. A car isschool. A car is
equipped with wheelsequipped with wheels
which spin backwards.which spin backwards.
As the wheels spinAs the wheels spin
backwards, they grip thebackwards, they grip the
road and push the roadroad and push the road
backwards.backwards.

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

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LawLaw
The reaction of a rocket isThe reaction of a rocket is
an application of the thirdan application of the third
law of motion. Variouslaw of motion. Various
fuels are burned in thefuels are burned in the
engine, producing hotengine, producing hot
gases.gases.
The hot gases push againstThe hot gases push against
the inside tube of the rocketthe inside tube of the rocket
and escape out the bottomand escape out the bottom
of the tube. As the gasesof the tube. As the gases
move downward, the rocketmove downward, the rocket
moves in the oppositemoves in the opposite
direction.direction.

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

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

19. Newton’s 3Newton’s 3rdrd
LawLaw
 The thing to do would be to take one of the tools fromThe thing to do would be to take one of the tools from
your tool belt and throw it as hard as you can directlyyour tool belt and throw it as hard as you can directly
away from the shuttle. Then, with the help ofaway from the shuttle. Then, with the help of
Newton's second and third laws, you will accelerateNewton's second and third laws, you will accelerate
back towards the shuttle. As you throw the tool, youback towards the shuttle. As you throw the tool, you
push against it, causing it to accelerate. At the samepush against it, causing it to accelerate. At the same
time, by Newton's third law, the tool is pushing backtime, by Newton's third law, the tool is pushing back
against you in the opposite direction, which causesagainst 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. 1st
law: Homer is large and
has much mass, therefore he
has much inertia. Friction
and gravity oppose his
motion.
2nd
law: Homer’s mass x
9.8 m/s/s equals his
weight, which is a force.
3rd
law: Homer pushes
against the ground and it
pushes back.