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Homework Help: Conceptual Momentum Question

  1. Jan 14, 2010 #1
    1. The problem statement, all variables and given/known data
    When a ball hits a wall and comes back, it originally had a momentum P. However, when the ball comes back, it has a momentum of -P (as in going in the opposite direction.) According to the law of conservation of momentum, since the ball had an impulse of -2P, the wall must have gained 2P. How do you explain what happened to the 2P since the wall doesn't move?

    2. Relevant equations

    3. The attempt at a solution
    Just saying, this isn't a homework question or anything so put this question on low priority XD.

    I was thinking that the wall does move but because of how large the mass of the wall is compared to the mass of the ball, we just don't perceive a movement. For example, in a building, when a ball hits the wall, the wall gains a momentum of 2P, but since the wall is attached to the building, the building as a whole gains a momentum of 2P, but since the building is attached to the ground, the entire earth gains a momentum of 2P, etc. and since Earth has an infinitely large mass compared to the ball, there is no change relative to the actual velocity.
  2. jcsd
  3. Jan 14, 2010 #2


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    Homework Helper

    That sounds about right, the actual movement is so small that is it is almost negligible.
  4. Jan 14, 2010 #3
    Thank you very much for answering the questions so quickly XD

    (Oh and that signature of yours is brilliant)
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