rd
3
Newton’s
Law of
Motion
Newton’s 3rd law of motion
 For every action, there is an equal and
opposite reaction.
Spring scales demo
Force pairs
 Forces always come in
pairs  equal and
opposite action / reaction
pairs.
 Force pairs are acting ON
TWO DIFFERENT
OBJECTS
 Action: Bug hits windshield
Reaction: Windshield hits
bug
Balanced Forces vs. Force pairs
REACTION
The tissue box is
pushing down on the
table.
Normal Force
ACTION
Balanced Forces
The table is pushing up
on the tissue box.
(Both forces acting on the
tissue box)
The tissue box is pulling
up on the Earth.
REACTION
The Earth is pulling
down on the tissue box.
ACTION
Weight
How does a fish swim?
 A fish uses its fins to push water backwards. But a
push on the water will only accelerate the water
backwards. According to Newton’s 3rd law, the water
reacts by pushing the fish forwards. Thus the fish is
propelled through the water!
Which of the following forces
is considered a force pair?
1. Action: Bat pushes baseball to the right
Reaction: Baseball pushes bat to the left
2. Action: Bat pushes baseball to the right
Reaction: Air resistance pushes baseball to
50%
50%
the left
1
2
Action / Reaction
 Which of the following forces is
considered a force pair?
Force pairs involve two objects!
Action – bat pushes baseball
to the right
Action – bat pushes baseball
to the right
Reaction – baseball pushes
bat to the left
Reaction – air resistance
pushes baseball to the left
While driving down the road, an unfortunate
bug strikes the windshield of a bus. Quite
obviously, this is a case of Newton's third
law of motion. The bug hit the windshield
and the windshield hit the bug. Which of the
two forces is greater: the force on the bug
or the force on the bus?
1.
2.
3.
4.
Force on the bug
Force on the bus
They are equal
Not enough
information
25%
1
25%
25%
2
3
25%
4
Trick Question!
Each force is the same size. For every
action, there is an equal ... (equal!). The
fact that the bug splatters only means
that with its smaller mass, it is less able
to withstand the larger acceleration
resulting from the interaction.
Compare
 BUG
F=
m·
 Bus Windshield
a
F=
m
·a
Compare
Forces are equal
m = 1000 kg
m = 0.01 kg
a = 200 m/s2
a = 0.002 m/s2
F = ma
F=2N
F=2N
Newton’s 3 Laws 
Hockey
 http://www.nbclearn.com/portal/site/learn/
science-of-nhl-hockey
A gun recoils when it is fired. The recoil
is the result of action-reaction force
pairs. As the gases from the gunpowder
explosion expand, the gun pushes the
bullet forwards and the bullet pushes the
gun backwards. The acceleration of the
recoiling gun is ...
1. greater than the acceleration
of the bullet.
2. smaller than the acceleration
of the bullet.
3. the same size as the
acceleration of the bullet.
33%
1
33%
2
33%
3
Answer
The force on the gun
equals the force on the
bullet. Yet, acceleration
depends on both force
and mass. The bullet
has a greater
acceleration due to the
fact that it has a smaller
mass. Remember:
acceleration and mass
are inversely
proportional.