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Homework Help: Elastic collision of neon atom problem

  1. Oct 31, 2008 #1
    1. The problem statement, all variables and given/known data
    A neon atom (m = 20.0 u) makes a perfectly elastic collision with another atom at rest. After the impact, the neon atom travels away at a 57.9° angle from its original direction and the unknown atom travels away at a -45.4° angle. What is the mass (in u) of the unknown atom? [Hint: You can use the law of sines.]

    m = neon = 20 u
    M = mass of unknown
    [tex]\theta[/tex] = 57.9°
    [tex]\phi[/tex] = 45.4°
    v = initial velocity of neon
    v' = final velocity of neon
    V = final velocity of unknown

    2. Relevant equations
    I've set up conservation of momentum equations for both directions, and the energy equation, but I'm stuck with more unknowns than equations

    3. The attempt at a solution
    mv = mv'cos[tex]\theta[/tex] + MVcos[tex]\phi[/tex]
    0 = mv'sin[tex]\theta[/tex] - MVsin[tex]\phi[/tex]

    using a momentum vector sum diagram and law of sines? I got

    not sure how that helps but I guess I can then express v' in terms of v, eliminating one variable

    no idea where to go from here
  2. jcsd
  3. Nov 1, 2008 #2


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    Homework Helper

    Everything looks OK so far I think. You can also use the law of sines to get the other "side" of the triangle. The problem is now just one of algebra. I would pick one equation (maybe the energy one), and substitute in for the variables you don't want and then isolate M.
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