Qualitative Collision Lab L2
Name __________________________________
Conservation of Linear Momentum
The Essential Question:
How does the total momentum of two carts before collision compare to the total momentum of two
carts after the collision? Is the answer true for all types of collisions?
The Theory:
The principle of Conservation of momentum: For a system of interacting objects, the total momentum
remains constant, provided no external resultant force acts on the system.
Total initial momentum = Total final momentum
For two-object collision, momentum conservation is easily stated mathematically by the equation:
m1v1 + m2v2 = m1v1’ + m2v2’
If external forces such as friction are ignored, the sum of the momenta of two carts prior to a collision
is the same as the sum of the momenta of the carts after the collision.
Remember that momentum is a vector.
 It holds true no matter what kind of forces the bodies exert on each other. They may be
gravitational forces, electric or magnetic forces, tension in strings, compression in springs,
attraction or repulsion.
 It does not matter whether the bodies stick together or scrape against each other or bounce apart.
They do not even have to touch. When two strong magnets repel or when an alpha particle is
repelled by a nucleus, conservation of momentum still holds. Collision between particles is a
very important tool in the study of nuclei and elementary particles.
 The law is not restricted to systems of only two objects; there can be any number of objects in
the system.
 The law applies to a galaxy as well as to an atom.
 The law applies to two dimensional collisions.
In today’s lab, we will first look at each collision qualitatively and write down observations and
sketches for each type of collision.
The relative speed will be shown with a velocity vector. The length is proportional to the speed.
Elastic Collision 1: Elastic with magnetic bumpers. Equal mass, one cart moving & one cart at rest.
Before the Collision
After the collision
v1 = +1
v2 = 0
Block 1 is traveling at a constant velocity
towards block 2. Block 2 is at rest. Both blocks
have the same mass (m=1).
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Qualitative Collision Lab L2
Name __________________________________
Elastic Collision 2: Equal mass and equal speed . Moving towards each other.
Before the Collision
After the collision
Block 1 is traveling at a constant velocity
towards block 2. Block 2 is traveling towards
block 1. Both blocks have the same mass (m=1).
Elastic Collision 3: Cart with 2 black masses moving slowly towards an unloaded stationary cart.
Before the Collision
After the collision
In your own word, summarize what happens in an elastic collision. Answer the essential question
for your observations of the elastic collisions.
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Qualitative Collision Lab L2
Name __________________________________
Inelastic collision setup: Change the collision by turning the carts so the Velcro bumpers face one
another. The carts should stick together after collision.
Inelastic Collision 1: Equal mass, one cart moving and one cart stationary.
Before the Collision
After the collision
v1 = +1
v2 = 0
Block 1 is traveling at a constant velocity
towards block 2. Block 2 is at rest. Both blocks
have the same mass (m=1).
Inelastic collision 2: Equal mass and equal speed, moving towards each.
Before the Collision
After the collision
Block 1 is traveling at a constant velocity
towards block 2. Block 2 is traveling towards
block 1. Both blocks have the same mass (m=1).
Inelastic Collision 3: Cart with 2 black masses moving slowly towards an unloaded stationary cart.
Before the Collision
After the collision
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Qualitative Collision Lab L2
Name __________________________________
Explosion or a collision in reverse: Your teacher will demonstrate how to set this up and create the
explosion.
Explosion 1: Equal mass.
Before the Collision
After the collision
Both carts are at rest. The trigger has not
exploded the carts.
Both carts have equal mass.
Explosion 2: Unequal mass
Before the Collision
After the collision
Re-read the essential question and answer it using information from your observations.
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Qualitative Collision Lab L2
Name __________________________________
Questions and Review
Define momentum.
Symbol - __________
Unit - ____________ Equation - ________________________
What is the law of conservation of momentum?
Write the equation for the conservation of momentum for 2 objects.
What is the difference between elastic and inelastic collisions?
Give an example of each and explain what is occurring. A sketch would be good
Elastic Collision
Inelastic Collision
How does mass affect velocity when momentum is constant?
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Qualitative Collision Lab L2
Name __________________________________
Practice Problems
1. Two objects, A & B, have identical velocities. Object A has 3 times the mass of object B. How
do the momenta of each object compare?
2. A car and a bus are traveling down the highway at 55 mph. Which has the greater momentum?
Justify your answer.
3. Saraphina throws a 2 kg medicine ball to Danielle. If the ball travels through the air at 3 m/s,
what is the momentum of the medicine ball?
4. If Danielle has a mass of 50 kg and is wearing frictionless roller-skates, what will her momentum
be after she catches the ball? (hint: Conservation of Momentum) Justify your answer.
6. (Challenge Question) What will Danielle’s velocity be after she catches the 2 kg ball traveling at
3 m/s?
7. Greg, who has a mass of 75 kg, is running north at 2.6 m/s. He accidentally collides with Tommy, who has
a mass of 125 kg and is not moving.
Which of the following statements describes how much momentum each person has before the collision?
A. Greg has a momentum of 130 kg • m/s north, and Tommy has no momentum.
B. Greg has a momentum of 195 kg • m/s north, and Tommy has no momentum.
C. Both Greg and Tommy have a momentum of 130 kg • m/s north.
D. Both Greg and Tommy have a momentum of 195 kg • m/s north.
8. What is the momentum of a metal disc with a mass of 1.5 kg sliding on a frictionless surface at 0.75 m/s?
A. 0.50 kg • m/s
B. 0.85 kg • m/s
C. 1.1 kg • m/s
D. 2.0 kg • m/s
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