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Experimental questions about the law of momentum

  1. Oct 25, 2008 #1
    if say I have two carts that is initially at rest, one of the cart has a plunger and the plunger is set so that it's against the other cart. When I release the plunger both carts will travel at a certain velocity.

    The law of momentum conservation says that the momentum of the initial system is the same as the momentum of the final system.

    As far as my understanding goes, the momentum before the what we can say "collision" is 0 as both carts has an initial velocity of 0, as they start from rest. While on the other hand the momentum after collision (after I release the plunger) is some number, as the velocity is not 0. So how is it possible that law of momentum holds here?

    p initial is not equal to p final

    I use p to denote momentum here.

    Can someone please help me to clear my understanding
  2. jcsd
  3. Oct 25, 2008 #2


    Staff: Mentor

    Remember that momentum is a vector quantity. So if one cart has a momentum of 2 kg m/s to the left and if the other cart has a momentum of 2 kg m/s to the right then the total momentum is 0.
  4. Oct 25, 2008 #3
    if you consider at my example here, would you consider this as an elastic or inelastic collision? also how about the kinetic energy before and after the collision?

    as far as I understand, before the collision the kinetic energy is 0 as there is no initial velocity
  5. Oct 25, 2008 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    Yes, that is correct- so this obviously is NOT an "elastic" collision. In order to conserve energy you would have to consider the work done in pressing in the plunger which turns into kinetic energy when it is released.
  6. Oct 25, 2008 #5
    okay, so on whatever type of collision it is (elastic or inelastic), the law of momentum conservation always holds.. however the kinetic energy before and after the collision is not always the same.. if the kinetic energy of before and after is the same then its elastic collision if not then it's inelastic. Am I right?
  7. Oct 26, 2008 #6


    User Avatar

    Staff: Mentor

    Right. :smile:
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