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Homework Help: Momentum equation derivation

  1. Apr 11, 2007 #1
    1. The problem statement, all variables and given/known data

    Derive a law of conservation of momentum for an isolated system consisting of two interacting particles.

    2. Relevant equations

    It says that "The law is derived by applying Newton's second law to each particle and Newton's third law to the system."

    3. The attempt at a solution

    I don't understand this at all.... if you could explain me with an example, that would be so great.....
  2. jcsd
  3. Apr 11, 2007 #2
    Newtons second law:


    Newtons third law:

    "Whenever A exerts a force on B, B simultaneously exerts a force on A with the same magnitude in the opposite direction." - "[URL [Broken]

    What would you do to combine the forces of two particles?
    Last edited by a moderator: May 2, 2017
  4. Apr 11, 2007 #3
    well, does that mean

    -ma=ma for representing the situation where two particles collide?

    but i still don't understand how to proceed tp get

    initial total momentum =final total momentum
  5. Apr 11, 2007 #4


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    Gold Member

    I think that for this problem it's easier to use the other version of Newtons second law:
    F = dp/dt (change in momentum)*
    Because of Newtons second law the total force inside a system will stay 0, so what does that say about the total change in momentum?
    *From this you can derive F = ma:
    F = dp/dt = d(mv)/dt = mdv/dt = ma
  6. Apr 12, 2007 #5
    all right, I think I got it. Thanx!
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