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Homework Help: Inclines and friction

  1. Dec 5, 2013 #1
    The figure below shows a block of mass m resting on a 20° slope. The block has coefficients of friction µs = 0.74 and µk = 0.52 with the surface. It is connected via a massless string over a massless, frictionless pulley to a hanging block of mass 2.0 kg.

    (a) What is the minimum mass m that will stick and not slip?
    (b) If this minimum mass is nudged ever so slightly, it will start being pulled up the incline. What acceleration will it have?

    fs= μs*N

    I got part A correct.
    m = 19.6 ÷ (9.8 * sin 20 + 0.74 * 9.8 * cos 20) = 1.93

    but part B is not, I tried to use the same equation but change the static friction to kinetic

    and got 2.4 as the mass

    then F= MA
    F = 2.4 (A)

    I made a FBD and came up with

    WME-fk - wsin[itex]\theta[/itex]=a(M1+M2)

    Is this on the right track?


    Attached Files:

  2. jcsd
  3. Dec 5, 2013 #2


    User Avatar

    Staff: Mentor

    But the question states that the mass remains at the value 1.93
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