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Momentum Conservation

  1. Apr 22, 2009 #1
    This is a conceptional problem I'm dealing with.
    when no external forces act on a system momentum is conserved, right?
    now when gravity acts on a system, and there is a collusion(elastic), lets say a mass M and an inclined plane(not attached to the ground) of mass 3M, while M is dropped from certain height and it hits the inclined plane(which causes the plane to move and M to bounce)
    now the duration of the collusion is so small, so before and after it the momentum is conserved both in the X direction and the Y direction in my opinion, cause although there was a force acting during the collusion, it's effect is so neglect-able,or am I wrong??
    I would like to have an explanation,
    Thanks in advanced.
    Dw
     
  2. jcsd
  3. Apr 22, 2009 #2

    Doc Al

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    Staff: Mentor

    Right. Whenever the net external force is zero, momentum is conserved.
    Right! Usually one can assume that the collision is of such short duration, that all other forces (like gravity) can be ignored. That's called the "impulse approximation".
     
  4. Apr 22, 2009 #3
    Oh great,
    one more thing,
    is the Normal force(during impact) is also an external force?
    I've just asked some one and he said it is, but It doesn't work well for me, so..is that so?

    Thank You so much Al!
     
  5. Apr 22, 2009 #4

    Matterwave

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    Normal force is external if you are considering the dropped mass. It is an internal force if you consider both masses. The normal force on the inclined plane by the table is external; however, it's balanced by Mg.
     
  6. Apr 22, 2009 #5

    Doc Al

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    It depends on how you define the system. The normal force is a contact force between two objects. As long as both objects are part of the same system, then any contact force between them is an internal force as far as that system goes.
     
  7. Apr 22, 2009 #6
    Thank You!
     
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