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Homework Help: Mass on incline will it slip or stick?

  1. Oct 7, 2008 #1
    1. The problem statement, all variables and given/known data
    Figure shows a block of mass m resting on a 20* slope. The block has coefficients of friction uS=0.80 and uK=0.50 with the surface. It is connected via a massless string over a massless, frictionless pulley to a hanging block of mass 2.0 kg.

    Part A:
    What is the minimum mass m that will stick and not slip?

    Part B:
    If this minimum mass is nudged ever so slightly, it will start being pulled up the incline. What acceleration will it have?

    2. Relevant equations
    Fnet=ma
    us=F/N
    3. The attempt at a solution

    I really have no idea how to do Part B, but I tried part A and was real confident with my answer but it turned out wrong. This is what I did.

    (2kg)(9.8)=19.6N for the hanging mass

    assuming that the Hanging mass = to the tension force on the block with mass m I then did this...

    19.6N=m(9.8)cos20*(.8)
    m=2.66kg and this is wrong

    Were did I go wrong?

    Thanks
     

    Attached Files:

  2. jcsd
  3. Oct 7, 2008 #2

    tiny-tim

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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi charan1! Welcome to PF! :smile:

    (Useful tip: don't bother to multiply everything by 9.8 … all the terms have 9.8, so just call it g, and cancel it at the end. :wink:)

    (oh … and have a mu: µ :smile:)


    I take it you were applying Newton's second law in the direction of the slope.

    That means that you must include all the forces … you left out the force due to gravity. :smile:

    (For part B, use µK instead of µS)
     
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