PhysicsLAB Lab
Air Track Collisions

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In this lab we will investigate the law of conservation of momentum with five different gliders on a low friction air track. You must remember that the system is extremely sensitive and should NOT be used carelessly or roughly.
 
 
Rules
 
Here are some very important rules:
 
  • When the gliders are not on the track rest them on the towels to insure that they are not scratched.
  • NEVER place the gliders on the track unless the air supply is running, and then only place them gently and vertically.
  • NEVER move them along the track unless the air supply is running.
  • When releasing a glider push it by applying LIGHT pressure at the sides of the glider, not from the top of the glider. When it is necessary to launch a glider, do so by allowing it to initially recoil from a spring bumper before it collides with another glider.
  • Slower velocities generally produce better results that higher velocities.
  • Do NOT try and adjust the "horizontal level" of the air track. Your track has been readied by your lab instructor and should not need any further refinements. Do NOT move the track!
 
A FLASH animation illustrating typical behavior of air track collisions is available here. To remove some uncertainty you may which to do a few "virtual experiments" before you start collecting data.
 
 
Equipment
 
To conduct the experiment you will need the following:
 
  • a computer
  • an air track with air supply
  • four gliders
  • two index cards
  • two photogates
  • one LabPro
 
Your experiments will be done using the software LoggerPro 3.1 which is located under programs -> math. The software will automatically detect the LabPro and its photogates.
 
Before beginning your experiments obtain from your instructor the masses of your four gliders. Record their values in kilograms below.
 
 
mass of red-glider #1 (one magnet, one bumper) 

mass of red glider #2 (one magnet, one bumper) 

mass of red-glider #3 (two bumpers) 

mass of gold glider #4 (two bumpers) 

 
Experiment #1: Inelastic Collision
 
Carefully measure the width of an index card, fold it in half laterally, and place it along the spine of red-glider #1. Do NOT place any cards along the spine of red-glider #2.
 
 
Turn on the air supply and place red-glider #1, with its card, between the first photogate and the air track's left bumper and place red-glider #2 at rest in the middle of the track with its magnet facing glider #1's magnet (you are permitted to LIGHTLY hold it in place). Make sure that glider #1 has its bumper towards the air track's left bumper and its magnet oriented towards glider #2.
 
The first photogate should be placed between the two gliders, and the second gate after the gliders have collided but before they reach the other end of the track. It is vital that the photogates and gliders be placed such that the initially moving glider is no longer in contact with your hand by the time it reaches the first photogate but finished moving through the first photogate by the time that it collides with the second glider. Try to place the 2nd photogate right after the glider which is initially at rest so as to minimize the distance over which friction can act before the glider reaches the 2nd photogate.
 
When you are ready, give red-glider #1 a gentle push so that it recoils off the air track bumper and moves through the first photogate, collides and sticks to red-glider #2, and the two gliders move together at a reasonable speed through the second photogate. If the combined gliders barely move, you didn’t push hard enough.
 
Each gate will provide you with two times: a 1 and a 0. By dividing the width of the index card by the difference between these two times you can determine the velocity of the glider(s) before and after the collision. This has been pre-programmed for you in the EXCEL workbook called 1-gliders.xls. You will repeat this experiment 3 times. Paste each trial's dataset from LoggerPro in its designated section of the EXCEL worksheet. Your instructor should be able to see the results of all three trials. Remember to save your file in your period folder as LastnameLastnamegliders.xls
 
What was the filename of your EXCEL spreadsheet?
 

How does the velocity calculated on your spreadsheet for the single glider compare to the pair of gliders after the inelastic collision?
 
According to your EXCEL sheet, what is the average percentage of kinetic energy lost during this type of inelastic collision? Reference the absolute value of the EXCEL sheet's value in column N. 

 
Experiment #2: Elastic Collision with Equal Masses
 
This time you will launch red-glider #3 and collide with red-glider #1. Both gliders will need index cards or EQUAL width. In this experiment the gliders will bounce apart instead of sticking together. So place the bumper end of red-glider #1 facing towards red-glider #3. At the end of each trial you will need to stop red-glider #1 before it collides with the air track's right bumper.
 
 
Red-glider #1 will begin at rest in the center of the track with its bumper end towards red-glider #3 (once again, you are permitted to lightly hold it in place before the collision). Red-glider #3 will be released by allowing it to recoil off the air track's left bumper. Red-glider #3 will pass through its photogate one time after which red-glider #1 will also pass once through its photogate as it rebounds from the collision. Stop red-glider #1 before it strikes the air track's right bumper. Observations are done when red-glider #1 passes through its photogate.
 
Each photogate should be as close as possible to the end of the air track while still allowing you room to handle the gliders. Make sure that neither glider is in contact with your hand when it reaches the photogate. Remember that slower speeds produce better results than higher speeds.
 
You may redo this experiment as many times as necessary so that you can confidentally answer the following questions.
 
How does the initial momentum of red-glider #3 compare to the final momentum of red-glider #1 after the collision?
 

Using the result of your previous answer, explain why the first glider should stop and while the second glider moves off with a velocity that is nearly the same as the first glider?
 

Would you expect this to occur if the glider masses were not equal?
 
In this video of Newton's cradle, explain the behavior of the cradle when three marbles are simultaneously released.
 

 
Experiment #3: Elastic Collision with Unequal Masses
 
This time you will use red-glider #3 and gold-glider #4. You will need to place cards into the spines of each glider.
 
 
In this experiment gold-glider #4 replaces red-glider #1 from Experiment 2. You should not need to make any further adjustments. Run the experiment 3 times and paste your data on the third sheet of the EXCEL workbook. Your instructor should be able to see the results of all three trials. Remember to resave your file.
 
According to your EXCEL sheet, what is the average percentage of kinetic energy lost during this type of inelastic collision? Reference the absolute value of the EXCEL sheet's value in column N. 

How does the percentage of kinetic energy lost during this type of collision compare to the percentage of kinetic energy lost in Experiment #1 (inelastic collision between red gliders)?
 
Explain why you you think the previous answer occured.
 

 
Prediction
 
Your instructor will now give you a cart of unknown mass. Without the assistance of any pre-programmed cells, you are to determine the unknown cart's mass by repeating Experiment 3. It is recommended that you export your data to the fourth tab of the EXCEL workbook so that you can keep copies of each trial's data. It is also recommended that you repeat the experiment at least three times. Your instructor should be able to see the results of at least three trials. Don't forget to resave your file.
 
 
When you have calculated your unknown glider's mass, call your instructor over to verify your calculations and find out the correct value. You are to calculate a percent error against your predicted mass versus its actual mass. Show all of your calculations on a printout of the fourth worksheet of your EXCEL file.
 
  unknown mass
trial (kg)
1
2
3
teacher certification of calculations 

actual mass (in kg) 

 
 
 
 
 
 
 
 
percent error 



When you have completed collecting your data and finished all of your calculations for the experiment, submit your results online. You should turn in your data/calculations and a printed copy of the last tab of the EXCEL sheet (your prediction data). Remember to put your names on EVERYTHING.

 
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