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Homework Help: Elastic collision of particles!

  1. Nov 9, 2012 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    conservation of momentum, m1v1i+m2v2i=m1v1f+m2v2f


    conservation of KE, ½m1v1i^2+½m2v2i^2=½m1v1f^2+½m2v2f^2

    3. The attempt at a solution

    first, i defined my variables. v1i=-v2i, v1f=-0.750v1i, v1f=0.750v2i

    I tried isolating V2f using the conservation of momentum equation, getting V2f=[-m1v2i+m2v2i-m1(0.750v2i)]/m2.

    Then I use the conservation of kinetic energy equation and plug in my new v2f value. expand it out, then try to simply everything to get an answer for m2. I'm really bad at long and complicated algebra, I often cancel out items I'm not allowed to, and don't cancel out when I should. I've tried around 7-8 times now, all with varying answers, none of them right.
  2. jcsd
  3. Nov 9, 2012 #2


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    Homework Helper

    Welcome to PF, Jaeeeger!
    I'm just an old high school teacher - used to keeping the notation simple. It might be worth a try that way. Rather than V2f, I used "W" for the final velocity of the nm.
    For conservation of momentum I wrote mv - nmv = -.75mv + nmW (1)
    For conservation of energy (cancelling all the 1/2's right off):
    mv² + nmv² = m(.75v)² + nmW² (2)
    It doesn't looks so bad that way, does it? Cancel all the m's to make it even better. I solved (1) for W and substituted into (2) to get an equation in one unknown.

    If you type in your work here (or scan and upload), we will make sure you get it right.
    For the ² symbol, just copy one of mine and paste in your post.
    More symbols to copy here: https://www.physicsforums.com/blog.php?b=346 [Broken]
    Last edited by a moderator: May 6, 2017
  4. Nov 9, 2012 #3


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    Staff: Mentor

    If you know the correct answer, it is a good idea to provide it so others working along can verify they are on the right track.
  5. Nov 9, 2012 #4
    The homework is online, I just know if I get it wrong.

    Okay thanks for the help. I'm fairly sure I just screwed up my algebra somewhere, I get n=-1, which is wrong.

    I solved (1) for W, then plugged that into the conservation of energy.
    My algebra just isn't as hot as it used to be.
    Last edited by a moderator: May 6, 2017
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