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Homework Help: Help on a Momentum Lab

  1. Oct 25, 2009 #1
    1. The problem statement, all variables and given/known data

    So this isn't necessairly a direct problem, but we are currently doing a momentum lab in my AP Physics B course and the lab is basically dealing with 4 cases of collision between two carts. In all cases we are to measure the velocity just before collision of the two carts and just after collision. We are using sonic detectors on both sides of the track to do this. In case 1, the carts are collided (they are nearly the same mass) with one cart sitting motionless on the track and the plunger cart hitting it going approx. 3.0 m/s. In case 2, the conditions are the same except an approx 1/2 kg mass is added to the passive cart. In case 3, the carts are the same except they are turned so the velcro sides face each other so they stick together after the plunger cart rams into the passive cart. The fourth case is the same except for the same mass is added once again.

    2. Relevant equations


    3. The attempt at a solution

    We are asked what type of collision is each case, and I have stated it is elastic for case 1 and 2, and completely inelastic in case 3 and 4....... but I don't think this is possible because no collisions are perfectly elastic or inelastic? I may have to ask my instructor on this question...

    The second question I have is regarding how well my results support the expected results, but I don't know what my expected results should be? Should my momentums be related in some way? Would this be percent error or percent difference?

    And what key concept is this showing? conservation of momentum?
  2. jcsd
  3. Oct 25, 2009 #2


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    Homework Helper

    What does "perfectly inelastic" mean?
    Elastic means no kinetic energy is lost, and this is not likely to be perfectly true, but it could well be pretty close. I would go with that prediction in the first two cases, and then use conservation laws to work out the expected speeds after the collision. Compare those with the experimental results. It would be interesting to compare the predicted kinetic energy with the actual to see how close they are. I think a % difference would be more appropriate because it could perhaps be applied to other situations where the masses are much larger or smaller.
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