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Chapter 12 Section 3 Notes
Force: a push or pull on an object
Force is not something that an object
possesses
An object only exerts force on
another object
But when that object exerts its
force, the second object always
exerts one back
Every force is part of a pair of forces
An interaction
We call this a Force Pair
One is called an Action
The other is a Reaction
Whenever one object exerts a force
on a second object, the second
object exerts a force that is…
◦ Equal in Magnitude
◦ Opposite in Direction
To every action there is an
equal and opposite reaction
Wall hits fist
Hey! Why don’t you try some on
your own?

When one object exerts a force on a second
object, the second object exerts an equal and
opposite force on the first object.

Forces are equal in size
and opposite in direction
◦


Hand against a wall.
Forces do not cancel if
they act on different
objects
Forces only cancel if they
act of the same object
Momentum: Property possessed by an object.
Directly proportional to mass of the object.
Directly proportional to velocity of the object
at that moment.
(instantaneous motion)
Momentum is abbreviated as p (Greek ‘rho’)
m is the symbol for mass
v is the symbol for velocity
Momentum is the product of the mass &
the velocity of an object
Calculate the momentum of a 12 kg object
traveling at 4 m/s.
P=?
m = 12 kg
v=4m
s
p = mv
p = 12 kg x 4 m
s
p = 48 kg x m = 48kgm
s
s
Hyperphysics: calculate various momenta
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1. Calculate the momentum of a 0.15 kg ball that is
moving toward home plate at a velocity of 40m/s.
2. Which has greater momentum, a 2.0kg hockey puck
moving east at 2.5m/s or a 1.3kg hockey puck moving
south at 3.0m/s?
3. A track athlete throws a 2kg discus into a field with
a velocity of 21m/s. What is the momentum of the
discus?
4. Calculate the momentum of a 700g ball that is
rolling down a ramp at 4.6m/s.
Hey! That looks like Newton’s 2nd Law.
Force = mass * acceleration
Acceleration is the change in
velocity over time
So Newton’s 2nd Law could read
Force = change in momentum
Time
p = mv
F = ma
Momentum
Newton’s Second Law
But we can use Dv for acceleration.
Dt
F = mDv = Dp
Dt
Dt
OR
F = Dp
Newton’s second law as change in
momentum over time.
Dt
Rearrange to a new form,
FDt = Dp Impulse MomentumRelationship



Momentum is the product of the velocity and
mass of an object
Momentum = mass X velocity
So, the bigger it is and the faster it goes the
greater the momentum.


In a closed system, the loss of momentum of
one object equals the gain in momentum of
another object
Therefore, momentum is conserved!

Bill Nye!