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Momentum conservation

  1. Oct 31, 2010 #1
    1. The problem statement, all variables and given/known data
    A ball of clay is thrown against a wall and sticks there. In this process, momentum is not conserved because the clay stops moving.



    3. The attempt at a solution

    Im thinking it's not because there is no velocity anymore for the clay? Is this right?
     
  2. jcsd
  3. Nov 1, 2010 #2
    momentum is not conserved because the reaction force is absorbed by clay ball to change ts shape .
     
  4. Nov 1, 2010 #3
    I think, Law of conservation of momentum is only applied when
    1. There is no external force acting on the bodies
    2. The bodies in the system are rigid

    The clay ball is not rigid and stick to wall. Here total energy of the system is conserved but the total momentum is not.
     
  5. Nov 1, 2010 #4
    correct. momentum is not conserved. in this case, since the wall is hinged to the ground/celling, a reaction force from this places is acting against the force that tries to move the wall.
     
  6. Nov 1, 2010 #5
    The earth was pushed backwards (tangentially by a very small amount) when the the clay ball was projected from something presumably anchored to it, and the earth was pushed forward (also by a very small amount) when it hit assuming the wall was anchored.
     
  7. Nov 2, 2010 #6
    Here we are considering a system of two bodies i.e. a wall and the clay ball. If we consider the motion of the earth, the system will be changed. Momentum will be conserved only if there is no external force acting on the system. The force may be small or large, tangentially or normally, if it is acting on the system the momentum will not conserved.
    In the proof of the Law of conservation of momentum we take (For the system of two bodies)
    Action force = -(Reaction force)
    If any third external force acts on the system, the magnitude and the direction of the action as well as reaction force changes (Depending on the magnitude and direction of external force)
    The change in force tends to change in velocity and momentum of the bodies. This force does not change the initial momentum of the system but changes final momentum. Thus the momentum is not conserved.
     
  8. Nov 2, 2010 #7
    So your ball and wall are in outer space. Then you can't anchor the wall. The clump of clay and the wall will move after collission.

    Or are you postulating that these are the only two things in your universe? Then momentum is not conserved; but the universe doesn't exist, so who cares?
     
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