MOMENTUM
Expectations
 Calculate the impulse applied to, and
momentum of a physical system

Demonstrate and apply the laws of
conservation of momentum in one
dimension
When you see this star…write the information down!
Suppose you were captured by an evil, crazy physics
teacher. She gave you the following choice:
Stand in front of a 1000 kg truck moving at 1 m/s
Or
Stand in front of a 1kg meatball moving 1000 m/s
What do you choose?
The meatball is very dangerous!
Beware! It isn’t very massive, but it’s
moving very fast!
Think about it; we’ll come back to it
later…
What
What
is momentum?
is momentum?
Think in terms of sports…
Momentum can be thought of as “how difficult it is to stop a
moving object”
Momentum
depends
on…
Momentum depends on…
Equation Alert: Momentum=(mass)(velocity)
p = mv
The little “p” is from the term progress defined as “the quantity of motion
with which a body proceeds in a certain direction.”
You’re
Mrs. Lessner…you’re
nuts! Wouldn’t
nuts!
their
Wouldn’t
momentum
their
be the be
same?
momentum
the same?
Let’s compare the kinetic energy and the
momentum of both objects…
hmmm…which is worse?
The meatball or the truck?
Clearly KE and momentum are not the
same!
Write down the example I do on the board
comparing the momentum of the two objects
Units for momentum problems
Units for momentum problems
Kg•m/s
mass
velocity
Conservation of Momentum
Conservation of Momentum
Momentum isn’t created or destroyed in a given
system.
It may be transferred from one object to
another.
Only an outside or external force will change
the total momentum
Conservation of Momentum
Conservation of Momentum
What does that mean to me?
The total momentum before a collision = the
total momentum after a collision
m1v1i = m2v2f
The same applies for a “recoil” velocity
Conservation of Momentum
Conservation of Momentum
Vocab Alert:
Closed system: one not affected by external forces
 Isolated system: completely isolated from environment
 System: Portion of the universe focused on for study.

Conservation of Momentum
Example: Conservation of Momentum
Calculate the recoil velocity of a 4.0kg rifle
that shoots a 0.05kg bullet at a speed of 280
m/s.
Vfr = -3.5 m/s
Because the rifle has a much larger mass, it’s
velocity will be much smaller than the bullet
Impulse
Impulse
Product of net force and time
Vector in direction of force
An impulse causes a change in
momentum
Units: -N•s
Why is there a negative sign?
Impulse
Impulse
Equation Alert:
Impulse = (Ft)
http://www.acs.psu.edu/drussell/
Impulse-Momentum Theorem
Impulse-Momentum Theorem
Equation Alert:
Ft = p
Impulse = change in momentum – so a change in force over time,
creates an impulse
Impulse-Momentum Theorem
Impulse-Momentum Theorem
Ft = mv
Impulse
Momentum
Coming to a stop sign
Coming to a stop sign
Change momentum of car by applying brakes
More force = less time
More time = less force
How does my change in momentum change in
the two situations above? Or doesn’t it?
How do Airbags work?
How do Airbags work?
Increase time, thereby reducing force
Sports Science
Demonstration: Impulse-Momentum
Two identical balls are dropped from the same height onto
the floor. In case 1 the ball bounces back up, and in case 2
the ball sticks to the floor without bouncing. In which case
is the impulse given to the ball by the floor the biggest?
1. Case 1
2. Case 2
3. The same
The impulse-momentum theory says that the impulse
that acts on an object is given by the change in the
momentum of the object, and this change is
proportional to the change in velocity. The ball that
sticks has a velocity of downward to zero, but the
velocity of the ball that bounces goes downward then
upward. This change in momentum is greater and
therefore has a greater impulse on it.
Impulse
Example 1: Impulse
A soccer ball (mass 0.425 kg), rolls
toward an attacking midfielder at 6 m/s.
The ball is shot toward the goal at a
speed of 26 m/s. Given that the kick’s
impact lasted for 0.008s, what was the
average force on the ball?
Collisions
Collisions
Skip #1 on your Guided Notes page for now. We’ll come back to it.
Elastic Collisions
Collisions
Elastic
Inelastic Collisions
Collisions
Inelastic
An inelastic collision is one where:
Momentum is still conserved
Kinetic energy is lost.
The lost kinetic energy will be transformed into other
energy types
The objects often interlock and stick together
May be deformed and mangled
Main indicator is the lack of conservation of KE.
Ex: car crash
Problem Solving Steps
Go back to “#1…” in your Guided Notes.
a.
Draw a picture for the “initial” scenario
◦ Before the collision
b.
Draw a picture for the “final” scenario
◦ After the collision
c.
d.
Write your Conservation of Momentum
Equation based on your initial and final
scenario
Isolate the unknown & substitute values
Example: Collisions
A 0.0050 kg bullet is fired at a
velocity of 450 m/s from a 2.0 kg
gun. What is the recoil velocity of
the gun? (explosions are reverse
collisions)
Elastic or Inelastic?
What is conserved: Momentum, KE or both?
A 0.0050 kg bullet is fired at a
velocity of 450 m/s from a 2.0 kg gun.
What is the recoil velocity of the
gun? (explosions are reverse
collisions)
mb=0.0050kg
vbf=450m/s
mg=2.0kg
vbi=0m/s
vgi=0m/s
Vgf in m/s
Initial
Final
𝑝𝑖 =
𝑣𝑔𝑓
𝑝𝑓
𝑚𝑏 𝑣𝑏𝑖 + 𝑚𝑔 𝑣𝑔𝑖 = 𝑚𝑏 𝑣𝑏𝑓 + 𝑚𝑔 𝑣𝑔𝑓
0 = 𝑚𝑏 𝑣𝑏𝑓 + 𝑚𝑔 𝑣𝑔𝑓
𝑣𝑔𝑓
−𝑚𝑏 𝑣𝑏𝑓
=
𝑚𝑔
−0.0050𝑘𝑔 × 450𝑚
𝑠
=
2.0𝑘𝑔
=-1.125m/s