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Inclined plane question

  1. Nov 1, 2006 #1
    A 3kg block starts at the top of a 30 degree incline and slides 2m down the incline in 1.5 seconds.
    a) Find acceleration of the block.
    b) Find the coefficient of kinetic friction between the blcok and the incline.

    For a) x = vot + 1/2at^2 --> a = 1.778m/s^2
    For b) *here's where i'm having some problems*
    I tried doing it two different ways and i get two different answers.

    Way 1:
    Ff = uFn
    Ff/Fn = u
    mgsinx/mgcosx=u
    tanx=u
    u = .577

    Way 2:
    ΣF=ma
    mgsinx - umgcosx = ma
    gsinx - ugcosx =a
    a-gsinx/-gcosx = u
    (1.778m/s^2 - 5m/s^2)/-(10*cos30) = u
    u = .372

    which way is wrong?
     
  2. jcsd
  3. Nov 1, 2006 #2
    So after reviewing what i typed in, i think that you can only use u = tanx when ΣF=0, is my assumption correct?
     
  4. Nov 1, 2006 #3

    radou

    User Avatar
    Homework Helper

    Way 1 is wrong, since the force of friction does not equal mgsinx.
     
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