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 m1v1+m2v2=(m1+v1)v' 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?