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Homework Help: Definition of collision

  1. Jan 31, 2005 #1
    In collision, law of conservation of momentum is applied only if no external force is added. Is weight considered as an external force? I think so. Anyone tell me ?

    If an apple falls from a tree and hit the earth. Is the weight considered as external force?
    Also, when they are in contact and starts to collide, the apple or the earth is back, in other words, the distance after impact is larger than that when it is at the moment of impact.
    However, the earth would have a force that pulling the apple coming towards it again after impact, and the velocity then is affected from the law of conservation of momentum and thus I consider weight is an external force,isn't it?
    Any advice is treasure
  2. jcsd
  3. Jan 31, 2005 #2


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    I'm not at all sure I understand what you are saying!

    What is or is not an external force depends upon what you are considering as part of the "system". If, in a problem involving a moving apple, you are considering only the motion of the apple, then gravity is an "external" force. The momentum of the apple is not conserved. If, in that same problem, you are considering the motion of both the apple and the earth (rather unusual) then gravity is an "internal" force and momentum (of the earth-apple system) is conserved.
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