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B Collision with magnets -- Elastic or Inelastic?

  1. Jun 5, 2017 #1
    In my experiment I am performing a collision with two carts with magnets attached with photogates and a labquest on an air track. I pushed the carts together and made sure that the carts did not come into contact. Based on my knowledge of collisions, I believed that the collision would have been elastic because the carts never come into contact and therefore kinetic energy would not dissipate during the collision. However, my data showed that neither momentum or kinetic energy was conserved. Does this mean it was an inelastic collision or did I just do something wrong? Any help is greatly appreciated. I have to do a presentation on this tomorrow.

    Extended: I came to the conclusion that the momentum loss was due to the fact that some momentum transferred to the magnetic field and friction also decreased the momentum of the two cart system. I also concluded that because magnetism is a conservative force, any change in kinetic energy has to be due to the force of friction which decreases the cart's velocity at all points between the photogates (not just during the collision). Are these conclusions reasonable? Thank you again to any one who can help
    Last edited: Jun 5, 2017
  2. jcsd
  3. Jun 5, 2017 #2
    To ask the obvious question: have you analyzed your friction losses? Since any real world setup has those, your math should take them into account.
  4. Jun 5, 2017 #3
    I did not do that actually. How would I use math to apply that to the kinetic energy or momentum in this collision? Would it make sense to call a change of kinetic energy from 36.2 J to 31.5 J negligible because of the friction in the system and therefore conclude that the collision was still elastic?
  5. Jun 6, 2017 #4


    Staff: Mentor

    Can you apply conservation of energy? You started with some potential energy. Where did it go?

    What is the potential and kinetic energy at the end of your experiment? Subtract those from the initial energy. The difference must have gone to frictional heating. Where else could it be?
  6. Jun 6, 2017 #5


    User Avatar
    Science Advisor

    Eddy currents?
  7. Jun 6, 2017 #6
    A 13% loss in KE is not negligible. There are a few issues with airtracks that you might consider:

    (1) Did you take the time to level the track? Depending on when the photogate measured the speed and when the carts collided, you might have gained/lost some velocity due to ##a=g\sin\theta## between measurements.
    (2) In the case where you're looking at magnetic damping, you're asking for 5 Joules to have been converted to (dissipated) current---or even radiation. Assuming that the carts are made of some low-resistivity material (i.e. metal), that's a lot of current. I suppose if the magnetic field near the metal changes fast enough that could occur.... but not likely.
    (3) Air tracks are notorious for producing air friction. This friction goes as the square the speed. Did you try the same experiment at slower speeds? This would be where I would put my money.
  8. Jun 6, 2017 #7
    Thank you for the help. I did not ensure that the air track was level so I can write about that in my lab report as something I could change. So I came to the conclusion that the observed decrease in momentum and kinetic energy was due to the heat lost due to friction between the photogates but not during the collision itself. Therefore, the collision was still elastic. I did not try the experiment at significantly different speeds but I did do some where the carts started from further away (100m instead of 50). When the carts started from further away I found that a greater percentage of kinetic energy and momentum dissipated (I'm assuming because friction acted for longer). Is this reasonable?
  9. Jun 6, 2017 #8
    Unfortunately I did not record the height or even check if the air track was level and I am not able to redo the experiment because of a lack of time. I will write about this in my lab report though as something I could have done better so thank you.
  10. Jun 7, 2017 #9
    Thank you for all of the help. I am presenting today and I feel adequately prepared so I do not need any further assistance.
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