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Homework Help: Two masses on an incline help

  1. Feb 27, 2008 #1
    1. The problem statement, all variables and given/known data
    Two boxes, with masses 12.5 kg and 25.5 kg, are connected by a taut string on a surface that is inclined at 25 degrees with respect to the horizontal. If the coefficients of kinetic friction between the boxes and the table are 0.100 and 0.150 respectively and the 25.5 kg box is placed below the 12.5 kg box on the incline, what is the magnitude of the acceleration of the 12.5 kg box just after they have begun to move down the incline?

    The answer we are looking for is supposed to be 4.14 m/s^2

    2. Relevant equations
    F = mg*sin(theta) - umg*cos(theta)

    3. The attempt at a solution
    I first assumed that since the coefficient of friction of the the heavier block was higher, it would have no bearing on the lighter block's acceleration. This yielded the answer 3.26 m/s^2, which was wrong.

    I then tried taking the forces produced by the blocks separately(12.5*9.81*sin25 - .100*12.5*9.81*cos25, and then the same but with the data for the heavier block), and added them together to get the overall force, and then divided by the sum of their masses yielding 2.96 m/s^2 as my answer, but to no avail.

    Lastly, I tried adding the separate forces again, but this time diving by just the mass of the lighter weight. Once again, wrong.

    I seem to of come to a roadblock in my thoughts, so any help would be appreciated.

    If something I've stated is unclear, I apologize, just tell me and I'll try to explain.
  2. jcsd
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