1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar
    Science Advisor
    Homework Helper

    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

    User Avatar
    Science Advisor
    Homework Helper

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook