Physics 1AL
CONSERVATION OF MOMENTUM
Winter 2009
1
Introduction
You have a summer job at Amtrak with a group examining the crash between two trains. Your
supervisor wants you to calculate the results of two different cases.
The first is a perfectly inelastic
collision where the two trains stick to each other.
The other case is an elastic collision where the two
trains bounce of each other with no loss of kinetic energy. Since the trains may be carrying different
cargo, their masses may be different. Your supervisor wants you to calculate the final velocity of the
trains as a function of the masses and initial velocities of the trains. You decide to calculate the resulting
velocities for the system and then build a laboratory model using gliders to check your calculation.
Your Objective:
To experimentally determine the final velocities of two objects after collision.
______________________________________________________________________________
Prelab questions:
Read
sections 6.1, 6.2, 6.3 in Serway & Faughn
Answer
each of the following questions in a few sentences of your own words:
1.
A 0.75 kg ball is thrown horizontally towards a wall with a speed of 15 m/s.
The initial
velocity is chosen to be the positive xdirection for this question.
The ball horizontally
rebounds back from the wall with a speed of 15 m/s in the negative xdirection.
What is
momentum of the ball before it hits the wall,
p
i
?
What is momentum of the ball after it
hits the wall,
p
f
?
What is the change in momentum of the ball,
Δ
p
?
(Give both
magnitude and direction for each of the previous answers.)
Is momentum conserved for
the ball?
2.
Perfectly Inelastic Collision
.
Draw diagrams showing a situation where a bigger,
moving train,
v
i
, collides with a smaller train that is not moving.
Assume that after the
collision the trains stick together.
Show separate diagrams for the situation just before
the collision and just after the collision. Assume the trains have different masses (
m
big
and
m
small
).
Make sure you identify your isolated system.
Write down the momentum
conservation equation for the scenario in question.
Solve for the final velocity of the
two trains,
v
f
, in terms of
v
i
,
m
big
, and
m
small
.
3.
Elastic Collision
. Draw two diagrams for the situation where a moving train collides
with a train that is not moving, and the trains use their springy bumpers to bounce off
each other without damage. One diagram should show the instant just before the
collision and the other the instant just after the collision. Assume the trains have
different masses (
m
moving
and
m
target
) Make sure you identify your
isolated system
. Solve
for the final velocity of each train
in terms of the initial velocity of the initially moving
train
and the two masses.
Hint: use conservation of momentum and the relation between
the relative velocities before and after collision that arises due to conservation of energy.
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 Summer '08
 ANDERSON
 Kinetic Energy, Momentum, Velocity, Inelastic collision, Elastic collision, final velocity

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