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Static friction preventing block from sliding

  1. Mar 23, 2009 #1
    1. The problem statement, all variables and given/known data

    If the coefficient os static friction between the incline and block is .3, then what is the minimum force,F, needed to prevent the block from sliding?
    m=20kg theta= 50 degrees

    2. Relevant equations

    Normal force= mgcosx
    Fs= uk mgcosx

    3. The attempt at a solution

    I know how to solve this this for the perpendicular force (up the ramp) but I cant seem to figure out how to do it for thr parallel force.
  2. jcsd
  3. Mar 23, 2009 #2


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    I assume that you drew yourself a diagram with a coordinate system that has its x-axis parallel to the inclined plane and its y-axis perpendicular to the plane. Then, what is the force due to gravity acting to move the block down the plane? What is the frictional force acting to oppose the motion down the plane? What is the condition on the sum of all forces along the plane so that the block does not move?
  4. Mar 23, 2009 #3
    Fore due to gravity is mgsinx and the frictional force = (coefficient of fiction)(mgCosx)
    And the condition on all forces along the plane so that the block doesnt move has to equal to zero

    But I cant seem to get what to set equal to what
    Last edited: Mar 23, 2009
  5. Mar 23, 2009 #4
    anyone? Im doing a test review for a test tomorrow so this would greatly help me
  6. Mar 23, 2009 #5


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    Homework Helper

    You have the right parts.

    Draw a diagram and add the forces that need to make it be static.
  7. Mar 23, 2009 #6
    ok so
    F + mgcosx= N
    (friction)N= mgSinx

    So F = (mgSinx)/(friction) - mgCosx

    There we go, thanks again
  8. Mar 23, 2009 #7


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    Homework Helper

    Not exactly.

    F + μmgcosθ = mgsinθ

    You only need to supply force in excess of what friction alone will not.
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