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Momentum and KE in multiple collisions at the same instant

  1. Dec 18, 2005 #1
    Hi there,
    i've been coding a simulator (ellastic collisions with billard balls) and i wanted to check if the results the program outputs are indeed valid.
    My doubts are with multiple collisions at the exact same instant and with kinetic energy as well.
    Just to make clear, a possible multiple collision with particles A, B and C could be:
    A hits C, and B hits C, at the exact moment
    The results i've ended with, is that the momentum is always preserved... independent of the type of collision (single or multiple collision)
    But that doesn't happen with KE when it's a multiple collision.
    example (particles A to D move towards X at the same velocity and are at the same distance):
    Initially the momentum = 0, and the KE = 8. But after the collision the KE = 0. And all the particles stay together.
    And another thing... aren't the laws of conservation of momentum only valids for collisions between 2 objects? If so, why does "my" momentum stays constant?
    Is this what is supposed to happen? :uhh:
    Or is there something i don't know about it?
    Thanks in advance
    ps: why aren't newlines supported? i have to use dots to space the text in order to make it clearer.
  2. jcsd
  3. Dec 18, 2005 #2
    No , momentum conservation is independent of number of objects undergoing collisions. Momentum conservation is applicable as long as there is no external force acting on the objects undergoing collision. For example , if the surface on which balls roll is not frictionless , the momentum is not conserved due to friction as an external force. K.E conservation takes place when no work is being done by any external force.

    K.E conservation is applicable in the direction where external force does zero work . K.E .C is a part of Energy conservation taking place at same potential energy.

  4. Dec 18, 2005 #3
    Even in inelastic collsiions momentum conservation is applicable because the deformation of one of the objects takes place due to impulse force of otherobject which are internal forces for the two balls.Still there is no external force.

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