Pre-lab Questions
What is mass?
What is the definition of momentum and its formula?
What is impulse?
If object A hits object B by what force does B hit A, explain?
How does impulse relate to the conservation of momentum?
What is an elastic collision?
What is an inelastic collision?
Why is an air track used in this investigation of momentum?
What is conserved in a perfectly elastic collision?
What is conserved in a completely inelastic collision?
Conservation of Momentum
Using Air tracks
Name: _________________________________________ Date: __________________ Block ______ Day ______
Introduction:
Collisions of two frictionless objects (gliders on a track, also referred to as carts or cars) can be described
in terms of momentum (and sometimes energy) conservation. There are three types of collisions: “elastic”
(kinetic energy is conserved), “inelastic” (kinetic energy is lost, note that collisions can be completely or
partially inelastic depending on whether they completely stick together or not), and “separation” (when the
masses that were together separate). The lab experiment below allows you to observe most of these types
of collisions and to test for the conservation of momentum and energy.
Understanding of these concepts is practical in everyday life whenever two or more objects interact,
whether during a sporting event, traffic accident or jumping from a tree.
Objectives:
1) During this lab investigation you will test for conservation of momentum in different interactions.
2) Measure and calculate the changes in energy for different interactions.
3) Classify each interaction as elastic, inelastic or explosion (or separation)
Equipment:
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Air track and air source
Air track cars 2 same mass, 1 different (Velcro bumpers and non-Velcro)
Electronic timers
Index cards and tape if necessary to use as a flag for the timers
Different car bumpers
Meter stick/ruler
Scale
Experiment 1: Determine the speed and momentum of car A
In this experiment you will understand how to use the timers and determine the speed and momentum of
the cars.
1) Measure and record the length and mass of the each car A, B and C.
2) Place car A on one end of the track near the air source.
Do not slide the car along the track unless the air is turned on.
3) Turn the photo gate timer switch on and set to interval mode with both lights A and B on. Note
which timer goes to photo gate A and B.
4) Give the car a gentle push and allow the car to pass through the photo gates and stop the car before
it returns back through the gates.
5) Record the times by pressing the A or B button. The time corresponds to the light that is on. With
both lights on the time between photo gates is displayed.
6) Calculate the speed of the car(s).
Car A
Car B
Car C
Mass (kg)
Length (m)
Calculate the speed and kinetic energy of the Car through Gates A and B, using the length of the car
above. (Show sample calculation)
Timer A
Timer B
Timer AB
Time (sec)
Distance (m)
Speed (m/s)
Kinetic Energy (J)
Are all of the speeds the same? Why or why not? Explain.
Experiment 2: Collision with stationary car of same mass with bumpers
1) Place car A on one end of the track near the air source and one at the midpoint between the
photo gates. Do not slide the car along the track unless the air is turned on.
2) Turn the photo gate timer switch on and set to interval mode with both lights A and B on.
Note which timer goes to photo gate A and B.
3) Give the car a gentle push and allow the car to pass through the photo gates colliding with the
stationary car.
4) Record the times.
5) Calculate the speed of the car(s), momentum and kinetic energy of the cars. Record in the data
tables.
Experiment 3: Collisions with stationary car of different masses with bumpers.
1) Place car A on one end of the track near the air source and Car C of different mass at the
midpoint between the photo gates.
2) Turn the photo gate timer switch on and set to interval mode with both lights A and B on.
Note which timer goes to photo gate A and B.
3) Give the car a gentle push and allow the car to pass through the photo gates colliding with the
stationary car.
4) Record the times.
5) Calculate the speed of the car(s) momentum. And use conservation of momentum to
determine the unknown final velocity of car A. Record your results in the data tables.
Experiment 4: Collisions with stationary car that sticks together.
1) Replace the bumpers or turn the cars so that the Velcro sides are toward each other.
2) Place car A on one end of the track near the air source and Car B of the same mass at the
midpoint between the photo gates.
3) Turn the photo gate timer switch on and set to interval mode with both lights A and B on.
Note which timer goes to photo gate A and B.
4) Give the car a gentle push and allow the car to pass through the photo gates colliding with the
stationary car, the cars should stick together, if not try again with a gentler push.
5) Record the times.
6) Calculate the speed of the car(s) momentum and kinetic energy. Record in the data table
Conservation of Momentum
2.1
2.2
2.3
Momentum
of Car B
Before
collision
(kgm/s)
0
0
0
3.1
3.2
3.3
0
0
0
4.1
4.2
4.3
0
0
0
Run
number
Momentum
of Car A
before
Collision
(kgm/s)
Momentum
of Car A
after
collision
(kgm/s)
Momentum
of Car B
after
collision
(kgm/s)
KE of Car A
after
collision
(J)
KE of Car B
after
collision
(J)
Total
momentum
before
collision
(kgm/s)
Total
momentum
after
collision
(kgm/s)
Ratio of
total
momentum
after/before
Conservation of Energy
Run
number
KE of Car A
before
Collision
(J)
2.1
2.2
3.1
KE of Car B
before
Collision
(J)
0
0
0
4.1
4.2
4.3
0
0
0
Total KE
before
collision
(J)
Total KE
After
collision
(J)
Ratio of
total KE
after/before
Analysis:
1) Next to each table identify which type of collision occurred.
2) Was the momentum conserved within acceptable experimental error in all trials? Explain.
3) Was the energy conserved in all of the experiments within experimental error? Explain.
4) In a Separation Experiment where the masses are together and then spring apart, would you
predict that the energy would be conserved? Create a mathematical example and prove your
prediction.