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Basic Statics Question

  1. Feb 28, 2007 #1
    1. The problem statement, all variables and given/known data
    A tile on a roof becomes loose and slides from rest down the roof. The roof is modelled as a plane inclined at 30 degrees to the horizontal. The co-efficent of friction between the tile and the roof is 0.4. The tile is modelled as a particle of mass 'm'kg.

    a) Find the acceleration of the particle as it slides down the roof.

    2. Relevant equations
    Fr(Friction Force)=uR
    Reaction force from the plane = mgcos(angle)
    Fr= 0.4mgcos(angle)
    The force of the particle sliding down the plane x = mgsin(angle)
    Total force going down the plane is x - Fr=ma

    3. The attempt at a solution
    R = mgcos(30)
    F = uR
    Fr= 0.4mgcos(30).

    Total force down the plane as it is sldiing down is:
    mgsin(30) - 0.4mgcos(30) = ma

    I can't solve this, as there are TWO unknowns, then theres the fact I can't find m. I've tried looking thorugh text books but no avail.

    This is my first post here =) I am attempting to complete as many practice examination questions throughout March and started early instead.

    I believe once I answer a), the rest will be easy as well (a needs to be answered for b and c).

    Thanks to all!
  2. jcsd
  3. Feb 28, 2007 #2

    Doc Al

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    Staff: Mentor

    Hint: Divide by m. :wink:
  4. Feb 28, 2007 #3


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    Homework Helper

    Are you sure you need to find m? :wink:

    Edit: too late, and too similar. :smile:
  5. Feb 28, 2007 #4
    Dear god, I feel embarressed.

    I have worked it out, THANK YOU.
    mgsin30 - 0.4mgcos(30) = ma

    All over m

    gsin30 - 0.4gcos(30) = a
    1.50 ... = a
    1.5ms^-2 = a

    Thank you all, I can resume studying now :)
    Such a speed reply!
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