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Homework Help: Conservation of Momentum

  1. Nov 9, 2006 #1
    Can someone help me to prove the law of conservation of momentum using an example?
  2. jcsd
  3. Nov 10, 2006 #2
    Hmm... right now I can't really think of any that actually PROVES it. But I can prove that it always works no matter what situation it is applied to. Just show a situation, e.g. car accidents, rockets, springs even.
  4. Nov 11, 2006 #3

    Andrew Mason

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    The proof is one line:

    [tex]F = dp/dt = 0[/tex]

    That is the definition of force. If the force acting is 0, dp/dt = 0 so p is constant.

  5. Nov 11, 2006 #4
    I don't actually thinks that actually proves it. That just defines force. Other than practical experiments, or actually showing it works in every situations, I can't really think WHY it must be so.

    Perhaps it is derived from elastic collisions, and kinetic energy is conserved. Perhaps not.
  6. Nov 11, 2006 #5

    Andrew Mason

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    Conservation of momentum follows from Newton's first law. Since:

    [tex]F = dp/dt[/tex]


    [tex]F = 0[/tex]

    then: P = constant.

    Why is Newton's first law true? It may not be. But to prove something, youi have to start with a premise. The premise is that F = dp/dt, Newton's first law. So far, no one has been able to show that it is not true.

    Last edited: Nov 11, 2006
  7. Nov 11, 2006 #6
    Ohhh... the problem of asking to "prove" versus "verify/demonstrate". :uhh:
  8. Nov 12, 2006 #7
    Thats proof by contradiction that is. Since you can't disprove it that means its right. I suppose you might accept it that way. But what I am going about is more that you can't proof by induction and you can't really explain WHY momentum is conserved.
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