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Board Pulled Out From Under A Box - friction, force and acceleration problem

  1. Nov 6, 2008 #1
    I have this problem about two masses, but im not really sure how to approach it..

    1. The problem statement, all variables and given/known data

    A box of mass m1 is sitting on a board of mass m2. The board rests on a frictionless horizontal surface. The coefficient of static friction between board and box is μs. The coefficient of kinetic friction, μd, is less than μs. Ff represents the magnitude of the friction force between the board and the box.
    Find Fmin, the constant force with least magniude that must be applied to the board in order to pull the board out from under the box (which will then fall off the other end of the board).

    2. Relevant equations

    F=ma
    Friction force=μmg

    3. The attempt at a solution

    I know that i am trying to find the force which needs to be applied to the board to overcome the static friction between the board and the box. I started by drawing force diagrams for the box and the board, but im not sure that theyre right.. Ive also formulated some equations for the forces and acceleration of the board and box. All my working is in the attached image.
    But now im stuck, I dont really know where to go from here... Can anyone give me a hint as to what to look at next?
    Thanks :smile:
     

    Attached Files:

  2. jcsd
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