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Homework Help: Inelastic Collision: Finding initial velocity of Rail Car who's final velocity = 0

  1. Nov 14, 2014 #1
    1. The problem statement, all variables and given/known data
    A Rail car with a mass= 27000kg's moving at 4m/s collides with another rail car with mass 80,000kg that was moving towards the first. After the collision, both cars come to a complete stop, what was the initial speed of the second rail car?

    2. Relevant equations

    3. The attempt at a solution
    This was a seemingly simple test question, but while most of the class got the same absolute value answer of 1.33m/s, half of us got -1.3m/s and the other half +1.3m/s. Logically the second car hand a negative momentum, but algebraically it seems like a positive answer is correct. Any clarification?
  2. jcsd
  3. Nov 14, 2014 #2


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    Science Advisor
    Homework Helper
    Gold Member

    You are asked for a speed, but the conservation law applies to velocities. If, as is generally advisable, you define the positive direction to be the same for all parallel velocities, displacements and accelerations then it will happen that some velocities will be negative. Speeds, however, are generally taken to be pure magnitudes, so are not quoted with negative values.
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