Train Moving pulling two boxcars

  1. 1. The problem statement, all variables and given/known data
    A train consists of a 4000 kg locomotive pulling two loaded boxcars, the first with a mass of 12,000 kg and the second with a mass of 16,000 kg. Assume that the boxcar wheels roll without friction and ignore aerodynamics. The acceleration of the train is 0.4 m/s2.

    With what force strength, in Newtons, does the first boxcar pull on the last boxcar?


    2. Relevant equations

    F=ma

    3. The attempt at a solution

    I thought the mass should be the total of the last two boxcars which would be 12000 plus 16,000 =28,000 so I tried 28,000*0.4m/s^2 which came out to 11,200 which is obviously wrong the answer to the question should be 6E3, what am I doing wrong?
     
  2. jcsd
  3. LowlyPion

    LowlyPion 5,337
    Homework Helper

    If the train is accelerating then what force is accelerating just the last car?
     
    Last edited: Feb 5, 2009
  4. PhanthomJay

    PhanthomJay 6,228
    Science Advisor
    Homework Helper
    Gold Member

    You should be using Free Body Diagrams (FBD). What you have calculated is the the pulling force of the engine on the first boxcar, not the pulling force of the first boxcar on the last boxcar. The easiest way to do this problem is to look at the last boxcar only in a FBD of that last boxcar. Identify the force acting on it, and solve for its value using Newton 2.
     
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