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A 'simple' collision

  1. Nov 27, 2006 #1
    Sorry that I don't have LaTeX, I'll do my best without it:

    A train with known mass collides with a stationary train and the two link together. If 27% of the initial kinetic energy is dissipated, what is the mass of the stationary train? We are given no velocities.

    M1 = mass of moving train (known)
    M2 = mass of stationary train
    Vi = initial velocity of moving train
    Vf = final velocity of the 2-train system
    Ki = initial kinetic energy
    Kf = final kinetic energy
    Wnc = work done by non-conservative forces
    Pi = initial momentum
    Pf = final momentum

    Using conservation of momentum:
    Pi = Pf
    (M1)(Vi) = (M1+M2)(Vf)

    Using conservation of energy:
    Ki + Wnc = Kf
    (1/2)(M1)(Vi)^2 - (.27)(1/2)(M1)(Vi)^2 = (1/2)(M1+M2)(Vf)^2
    (.73)(M1)(Vi)^2 = (M1+M2)(Vf)^2

    ...2 equations, 3 unknowns. What am I forgetting?
     
  2. jcsd
  3. Nov 27, 2006 #2

    OlderDan

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    That you were not asked to find the velocities. Perhaps they are not unique. Does that give you any ideas?
     
  4. Nov 27, 2006 #3
    I can't see that it does. I realize the velocity of the trains after impact is the same, that they become a system. I've already taken that into account, though.

    Help?
     
  5. Nov 27, 2006 #4

    OlderDan

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    Try dividing your two equations
     
  6. Nov 27, 2006 #5
    Righteous, dude. Can't believe that one slipped me by. I get so stuck on comparing numbers of equations and unknowns that I miss the little things. Thanks much.
     
  7. Nov 27, 2006 #6

    dextercioby

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    But we do have LaTex. All you need next time is write the code between [ tex ] [ /tex ] tags and it comes out nicely.

    Daniel.
     
  8. Nov 27, 2006 #7
    Oh, great. Where can I go to find out how to write the code?
     
  9. Nov 27, 2006 #8

    dextercioby

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