Chapter 2 – Lesson 4
Newton’s Third Law of
Motion
When one object exerts a force on a second
object, the second object exerts an equal
force in the opposite direction on the first
object.
ANOTHER WAY TO SAY IT….
For every action there is an
equal and opposite reaction.
Think about it…
How is the gymnast able to flip over?
When the gymnast PUSHES the vault, the vault
PUSHES BACK against the gymnast.
The LENGTH of the force arrows are the SAME
but the direction is OPPOSITE
Force Pair
The forces two objects apply to each other.
Force pairs will always act in OPPOSITE
DIRECTIONS
The girls feet act on THE BOAT
The boat acts on THE GIRLS FEET
Do Action-Reaction Forces
Cancel?
If 2 equal act in opposite directions they are
balanced and cancel each other out. There is no
movement
Action – Reaction forces DO NOT cancel out
because they are acting on 2 DIFFERENT OBJECTS
FORCE PAIR
If one of the pairs is much more MASSIVE you
will only see the LESS MASSIVE object MOVE
Example – When you
push down on the Earth,
you won’t see the Earth
move, only you jumping
in the air
ACTION AND REACTION
 In a FORCE PAIRone force is called the
ACTION FORCE and the other force is
called the REACTION FORCE
For every ACTION FORCE there is a
REACTION FORCE that is EQUAL in strength,
but OPPOSITE in direction.
Action – Reaction forces don’t cancel
because they ACT ON DIFFERENT OBJECTS
How do Action-Reaction
forces work when you are…
JUMPING
• When you jump, you push down on the ground.
• The ground then pushes up on you. It is this upward
force that pushes you into the air.
How do Action-Reaction
forces work when…
A rocket launches
*When the rocket fuel is
ignited, a hot gas is
produced. As the gas
molecules collide with the
inside engine walls, the walls
exert a force that pushes
them out of the bottom of the
engine.
• The action force is - THE DOWNWARD PUSH
•The reaction force is - THE UPWARD PUSH ON THE
ROCKET ENGINE BY GAS
MOLECULES
Why doesn’t the bowling
ball move backward if the
pin is pushing on it equally?
The bowling ball has more MOMENTUM
Momentum
The measure of how hard it is to stop a moving
object.
Momentum = MASS x VELOCITY
P= m
x v
Momentum is a vector quantity
Units: kg
Mass =
m/s
Velocity =
kg * m/s
Momentum =
Momentum
More MASS = more MOMENTUM
More VELOCITY
= more MOMENTUM
More MOMENTUM = GREATER FORCE needed
to STOP an object.
Which object would have more momentum?
Practice Problem 1
What is the momentum of a bird
with a mass of 0.018 kg flying at 15
m/s?
Practice Problem 2
A golf ball travels at 16m/s, while a baseball
moves at 7 m/s. The mass of the golf ball is
.045 kg and the mass of the baseball is 0.14
kg. Which has greater momentum?
Law of Conservation of
Momentum
The total momentum of a group of objects stays
the same unless outside forces act on the objects.
The cue ball has MOMENTUM because it has
mass and velocity.
When it hits the other balls , the cue balls VELOCITY
& MOMENTUM decrease.
The other balls start moving (and have mass)
which mean they now have MOMENTUM
Stopping an object with
momentum
 To stop an object we need to apply a force over
a certain period of time. We call this impulse
 Impulse – the force acting on an object in a
specific amount of time.
IMPULSE = F x
 Force (F) = N
 Change in time ( t) = s
 Impulse (I) = N s
t
Using Newton’s 2nd Law to determine how
hard it is to stop a moving object.
 Newton’s 2nd Law tells us that F=ma
 fv  iv 
 We could also look at this as: F  m  

 t 
 And if we were to manipulate this formula by
multiplying both sides by time it would end up
looking like:
F  t m  ( fv  iv)
or
F  t  m  v
Impulse = change in momentum

Why does an egg break
or not break?
 An egg dropped on a tile floor breaks, but an
egg dropped on a pillow does not. Why?
FΔt= mΔv
In both cases, m and Δv are the same.
If Δt goes up, what happens to the force?
Right! Force goes down. When dropped on a
pillow, the egg starts to slow down as soon as
it touches it. A pillow increases the time the
egg takes to stops.
Practice Problem
 A net force of 100 N is applied to a 20kg cart that
is already moving at 3m/s. The final speed of the
cart was 8m/s. For how long was the force
applied?
F  t  m  v
100N  t  20kg  5m /s
100N  t  100kg  m /s
 Identify the variables
Mass = 20kg
Δvelocity = 8m/s-3m/s = 5m/s
Time = ?
Force = 100N


Practice Problem
A .057 kg tennis ball falls on a tile floor. The ball
changes velocity from -1.2 m/s to +1.2 m/s in 0.02 s.
What is the average force on the ball?
Identify the variables:
Mass = 0.057 kg
Δvelocity = +1.2 – (-1.2) = 2.4 m/s
Time = 0.02 s
using FΔt= mΔv
F x (0.02 s) = (0.057 kg)(2.4 m/s)
F= 6.8 N
Collisions
Objects collide in two different ways:
1. Elastic collision – When colliding objects
bounce off each other
2. Inelastic collision – When objects collide
and stick together.
The amount of momentum involved before the
collision will always be the same after the
collision.
Collisions with 2 moving
objects
When two objects are moving in the SAME
DIRECTION and aCOLLISION occurs, the
momentum of the slower object SPEEDS UP
and the momentum of the faster object SLOWS
DOWN
BEFORE
COLLISION
4 m/s
AFTER
COLLISION
2 m/s
Collisions with 1 moving
object
When ONE object is moving and COLLIDES into
a nonmoving object, all the MOMENTUM is
TRANSFERRED to the NONMOVING object.
BEFORE
COLLISION
4 m/s
AFTER
COLLISION
0 m/s
Collisions with connected
objects
When one object is moving and COLLIDES ,
but CONNECTS to a nonmoving object, the
MOMENTUM gets evenly split between the
TWO MOVING OBJECTS
BEFORE
COLLISION
4 m/s
AFTER
COLLISION
0 m/s