Chapter 5
Newton's Third Law
of Motion
1. FORCES AND INTERACTIONS
Force is not a thing but is an
interaction between one thing
and another.
Examples: 2 people pushing each
other on skate boards, driving a
nail, punching a bag
No single isolated force
Demo - Bathroom Scales
2. NEWTON'S THIRD
LAW OF MOTION
Law 3 - Whenever one object
exerts a force on a second
object, the second object exerts
an equal and opposite force on
the first.
Action-reaction pairs never act
on same object.
Defining Your System
Two objects define a system for a
Newton’s third law interaction.
We are not considering (necessarily)
the net force acting on an object.
An object cannot exert a force on
itself to cause an acceleration.
Action and Reaction on Different Masses
Consider you and the earth
Action: earth pulls on you
Reaction: you pull on earth
Recoil
F= a
m
F= a
m
Video – Scooter Propulsion
Cannot touch without being touched
Reaction: road pushes on tires
Action: tires push on road
Reaction: gases push on rocket
Action: rocket pushes on gases
3. SUMMARY OF NEWTON'S
THREE LAWS
Law 1 (Law of Inertia) Every object
continues in its state of rest, or of
uniform motion in a straight line,
unless it is compelled to change that
state by forces impressed
upon
it.



Law 2

F  ma
Law 3 - Whenever one object exerts
a force on a second object, the
second object exerts an equal and
opposite force on the first.
Chapter 5 Review Questions
A bug and a car collide. Which
experiences the greater force?
(a) bug
(b) car
(c) neither, they both experience the
same magnitude of force
Consider hitting a baseball with a bat.
If we call the force applied to the ball
by the bat the action force, identify
the reaction force.
(a) the force applied to the bat by the hands
(b) the force applied to the bat by the ball
(c) the force the ball carries with it in flight
(d) the centrifugal force in the swing
4. VECTORS
Imagine that you have a map that leads
you to a buried treasure.
This map has instructions such as
15 paces northwest
of the skull.
The 15 paces is
a distance and
northwest is a direction.
N
Quantities that require a
magnitude and direction for
specification are called vectors.
Those quantities that have no
direction are called scalars.
Examples of scalars in physics are
mass
time
distance
density
work
energy
Examples of vectors
displacement
acceleration
momentum
in physics are
velocity
force
angular momentum
The math associated with scalars
is familiar to everyone.
The math associated with vectors
is more involved.
Let’s explore the graphical
addition of vectors.
Let’s use a treasure map again as an
example of the addition of vectors.
Let’s imagine the instructions tell
you to go 4 miles east then 3 miles
north.
5 miles
3 miles
36.90
4 miles
In this case you could have gone 3
miles north first and then 4 miles
east next and still end up at the
same location.
Your final position is 5 miles at
36.90 north of east.
It would have saved time if that
had been the one distance and one
direction traveled in the first
place.
We say that the 5 miles at 36.90
north of east is the vector sum of the
4 miles east vector and the 3 miles
north vector.
The order of the addition does not
matter.
Examples of addition of vectors
follows. The method used will be the
head-to-tail.
Force Vectors
What is the resultant force?
3 Newtons north
5 Newtons @
370 north of east
4 Newtons east
Velocity Vectors
What is the actual velocity?
80 km/h north
wrt the wind
100 km/h @
530 north of east
Wind at 60 km/h east
Components of Vectors

Consider the vector R
y

R

Ry

Rx
x
Velocity Components in Projectile Motion