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Conservation of Momentum with Collisions

  1. Feb 19, 2015 #1
    1. The problem statement, all variables and given/known data

    A diesel engine weighs 8 times as much as a freight car. This diesel engine, moving on a horizontal track at 3 km/h, crashes into the freight car (which was moving at 2 km/h in the same direction). What is the velocity, in km/h, of the now linked together engine/car combination? You may assume no numerical values not given.

    2. Relevant equations

    V= mdvd + mfvf over M or md + mf

    The sub d is for diesel engine
    The sub f is for freight car
    3. The attempt at a solution
    I am not really sure is the m's cancel and all that is left with is the v's. If so the final velocity would be 5 km/h
  2. jcsd
  3. Feb 19, 2015 #2
    Try calling the mass of the car m and then write the mass of the engine as 8m. You might be able to see what ends up canceling afterward.
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