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Homework Help: Trains colliding and latching together

  1. Oct 18, 2011 #1
    1. The problem statement, all variables and given/known data
    A train car with mass m1 = 550 kg is moving to the right with a speed of v1 = 7.2 m/s and collides with a second train car. The two cars latch together during the collision and then move off to the right at vf = 4.7 m/s.2)What is the mass of the second train car?
    2. Relevant equations

    momentum before = momentum after.

    kinetic energy before does not equal kinetic after.

    3. The attempt at a solution

    okay, I assumed the second car is not moving and solved for v final using this:

    m1v1+m2v2=(m1+m2)v final.

    My answer was wrong. Any ideas. My guess is that I shouldn't assume the second train car isn't moving but I don't know another way to solve it in that case.
  2. jcsd
  3. Oct 18, 2011 #2
    How do u know that your answer is incorrect i attempted your problem your equation m1v1+m2v2=(m1+m2)vf appears correct to me it gives 550*7.2+0=(550+m2)4.7 and I got an answer of 292.55 kg.
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