Do Now:
Who was the best teacher you ever had?
 What was it about that person and their class
that made an impact on you?

Brainstorming
What does the word “momentum” mean
to you?
 What about “impulse?
 How have you used these two words in
your life?

Do Now (1/3/12):
What does the word
“momentum” mean to you?
2. What about “impulse?
3. What was the best thing about
your break?
Happy New Year 
1.
Momentum
1/3/11
LAST TOPIC OF THE SEMESTER!!!
Linear Momentum:

Linear Momentum: the product of the
mass and velocity of an object; represented
by the symbol p (units: kg x m/s); is a vector
quantity
p  mv
Example:

Anquan Boldin has a mass of 130 kg. If he
runs east at 35 m/s, what is the magnitude
and direction of his momentum?
Example:

Anquan Boldin has a mass of 130 kg. If he
can run at the same speed as Haloti
Ngata, who has a mass of 200 kg, who has
the greater momentum?
Impulse-Momentum Theorem:

The impulse on an object is equal to the
change in momentum it causes
Ft  mv  mv f  mvi
Ft  p f  pi
Example: Stopping a vehicle
A 2200 kg SUV traveling at 94 km/h (26
m/s) can be stopped in
a. 21 s by gently applying the brakes
b. 5.5 s in a panic stop
c. 0.22 s if it hits a concrete wall
Find the average force exerted in each case.

Practice:
Use the rest of class to work on “Intro to
Momentum.” It is due tomorrow!
 #4 = “km/h” should be “m/s”

Do Now (1/4/12):
1.
2.
A 2400 kg SUV and a 1300 kg sports car
are traveling at the same speed. Which
one has the greater momentum?
If they are both traveling at 20 m/s, what
is the momentum of each?
Pass In:
Momentum homework (half-sheet)
 Last week’s Do Now’s (if you haven’t
already)
 Home Alone Extra Credit (if you haven’t
already)

Collisions and Conservation of
Momentum
1/4/11: Inelastic Collisions
Systems:
Closed system: a system which does not
gain or lose mass
 Isolated system: a system with a net
external force equal to zero

The Law of Conservation of Momentum

The momentum of any closed isolated
system does not change
pi  p f
Types of Collisions:
Elastic: objects do not stick together after
collisions
 Inelastic: objects stick together after
collision
 http://www.physicsclassroom.com/mmedi
a/momentum/creti.cfm

Inelastic Collision

Take a collision between two objects (m1 and
m2). Use Conservation of Momentum:
pi  p f

Objects sticking together after collision will
have the same velocity:
m1v1i  m2v2i  m1v f  m2v f
m1v1i  m2v2i  (m1  m2 )v f
Example:

A 1875 kg car going 23 m/s rear ends a
1025 kg compact car going 17 m/s on ice
in the same direction. The two cars stick
together. How fast do the two cars travel
together after the collision?
Elastic Collisions

Objects do not stick together; the objects
do not have the same final velocity
m1v1i  m2 v2i  m1v1 f  m2 v2 f
Example: #2 on back of homework:
Work with your seatmate to list the
variables in this problem.
 Determine whether the collision is elastic
or inelastic.

Practice:

Use the rest of class to work on the
worksheet “Collisions and Conservation
of Momentum.”
Elastic Collisions
1/5/11
Do Now:
A 1875 kg car going 23 m/s rear
ends a 1025 kg compact car initially
at rest on ice in the same direction.
The two cars stick together.
1. What type of collision is this?
2. How fast do the two cars travel
together after the collision?

Recoil:

Both objects start out at rest (both vi are 0)
m1v1i  m2 v2i  m1v1 f  m2 v2 f
0  m1v1 f  m2 v2 f
 m1v1 f  m2 v2 f
Examples of recoil:
Explosion
 A diver shooting a gun in the water
 Ice skaters pushing one another
 An astronaut throwing something in space

Example:
An astronaut at rest in space fires a
thruster pistol that expels 3.5 kg of
gas at 875 m/s.The combined mass of
the astronaut and the pistol is 84 kg.
 How fast and in what direction is the
astronaut moving after firing the
pistol?

Practice:
Please use the rest of class to work
on one of three things:
1. Your homework
2. Your notecard for your quiz
tomorrow
3. Your final materials list (if needed)

Do Now:

Two ice-skaters are at rest on the ice. The
ice skater with a mass of 70 kg pushes 50
kg skater, who recoils with a speed of -12
m/s. How fast and in what direction is
the70 kg skater moving?
Classwork:
Please work on one of three things only:
 Homework
 In classroom textbooks: pick 4 problems
from each set:
◦ P. 218: #22-28
◦ P.219 #34-40

Work on your notecard!
Do Now:
1.
2.
3.
What two types of collisions are there?
What are the steps for solving
momentum problems?
Turn in your hw and Do Now’s (you
should have six)!!!
Steps: Sketch the Problem

Sketch the problem before and after the event
using vectors, including an axis indicating the
positive and negative directions
Vi=26 m/s
Vf=0 m/s
+
Steps: List Knowns and Unknowns:
Knowns:
m=2200 kg
vi=26 m/s
vf=0 m/s
∆t=21s, 5.5s, 0.22s
Unknowns:
F=?
Steps: Calculation
Determine the momentum before and
after
 Apply impulse-momentum theorem to
calculate force
