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Conservation of momentum of ball thrown

  1. Jun 20, 2009 #1
    If say, a object such as a ball with velocity v m/s and mass m kg collides with a stationary solid wall, and the ball rebounds back with a velocity of v' m/s while the wall remains stationary, is momentum conserved?

    Since the wall is stationary, I would assume that the initial and the final momenta of the wall are zero. Hence applying the law of conservation of momentum,
    p initial = p final

    But then the velocity of the ball is in the opposite direction after the impact, so how would I justify the conservation of momentum in this case?
    Also, would the collision be elastic? (assuming the wall does not deform)
  2. jcsd
  3. Jun 20, 2009 #2
    No the collision is partially inelastic at has to be as the COR isnt 1, in all collisions the momentum is conserved. As the wall is very large it doesnt appear to move (it also only moves locally, by deformation).
  4. Jun 20, 2009 #3


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    Hi merry! :smile:

    By good ol' Newton's second law , total momentum in a direction is only conserved if there is no external force in that direction.

    So collisions in space, or collisions on a frictionless surface (where the only external forces are normal to the motion), obey conservation of momentum,

    but your wall has (I assume :wink:) a horizontal force keeping it in place, so horizontal momentum is not conserved. :smile:
  5. Jun 21, 2009 #4
    Hehe XD that makes sense! Thank you! =D
  6. Jun 21, 2009 #5


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    Of course, if you consider the momentum of whatever is holding the wall in place, momentum would be conserved. So if you could somehow measure the momentum of the wall, the Earth, etc. very precisely, you should find that the total momentum is conserved. (Actually, even then there are external forces, like gravity... but if there weren't, then you should find that the total momentum would be conserved ;-)
  7. Jun 23, 2009 #6
    That is an interesting idea o_O Thank you =D
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