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What to do…
Turn in any late or missing work.
Write and answer the following question in your journal.
Raise your hand when you are finished. You have 5
minutes!
A force of 63 N is applied to a 7 kg. mass. What is the
acceleration of the mass? Remember a = F/m
A quick Review
Tell your partner Newton’s 1st and 2nd Laws
Use your notes if you have to 
Newton’s 1st Law
An object at rest tends to stay at rest and an
object in motion tends to stay in motion unless
acted upon by an unbalanced force.
Newton’s 2nd Law of Motion
The acceleration of an object is equal to the net
force exerted on the object divided by the
object’s mass.
Force = Mass x Acceleration
Force
Acceleration = Mass
The acceleration due to gravity on earth = 9.8 m/s2
Newton’s Third Law
Chapter 20, Section 2
Turn to page 36 in your manual
Newton’s Third Law of Motion
If one object exerts a force
on another object, then the
second object exerts a force
of equal strength in the
opposite direction on the
first object
“For every action there is an
equal but opposite reaction”
Action – Reaction Pairs
Examples
● Gymnast pushes down on vaulting horse, the horse
pushes him up and over
● Kayak paddle pushes the water back, the water pushes
the kayak forward
● A dog pushes down on the ground, the ground pushes
up on the dog causing it to leap into the air
Other Examples…?….
Detecting Motion
●You can’t always detect motion when paired
forces are in action.
●If one of the pairs is 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
Do Action – Reaction Forces Cancel?
If 2 equal actions 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
Momentum
*A characteristic of a moving object that is related to
the mass and velocity of the object.
*Momentum = mass(kg) X Velocity(m/s)
*Like velocity and acceleration, momentum has a
direction. Its direction is the same as its velocity
Momentum
More momentum = harder to stop
More mass = more momentum.
You can stop a baseball traveling 20m/s, but not a
truck
The greater the velocity an object has, the harder it is
to stop. An arrow shot from a bow has a large
momentum even though it has a small mass
Conservation of Momentum
Conservation – same amount before and after
an event
Total amount of momentum objects have when
they collide is conserved
Law of Conservation of Momentum – states that
in the absence of outside forces, the total
momentum of objects that interact do not
change
PUT IT TOGETHER. . .
Continue with our explanations
of Newton’s laws with a picture
of the 3rd law.
Test Friday!
Complete the Physics Study Guide Part 2
by Wednesday 9/14