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Static Friction- Train stops, boxes move?

  1. Sep 21, 2010 #1
    1. The problem statement, all variables and given/known data
    The floor of a railroad flatcar is loaded with loose crates having a coefficient of static friction of 0.25 with the floor. If the train is initially moving at a speed of 48 km/h, in how short a distance can the train be stopped at constant acceleration without causing the crates to slide over the floor?


    2. Relevant equations
    F = ma
    f(s)max= u(s)N
    final v^2-init. v^2 = 2a*s(distance)
    Newton's second law?

    3. The attempt at a solution
    I've been cracking my skull on this one... I search and found this https://www.physicsforums.com/showthread.php?t=186380 however, I couldn't figure it out.
    Is the maximum deceleration -.25g?


    But when I plug that into: 0-13.33=2*a*s => -13.33/(-2*.25*9.8) = 2.72m and obviously something is wrong...


    Thanks in advance.
     
  2. jcsd
  3. Sep 21, 2010 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    You forgot to square the speed v=13.33 m/s.


    ehild
     
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