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Homework Help: Find Frictional force on a block

  1. Oct 4, 2011 #1
    1. The problem statement, all variables and given/known data

    A block of mass 9 kg rests on a plane inclined
    at an angle of 30◦. The static coefficient of
    friction between the block and the plane is
    0.7.
    What is the frictional force on the block?
    1. 9.36841 N
    2. 76.3834 N
    3. 53.4684 N
    4. 44.1 N
    5. 30.87


    2. Relevant equations
    Ff=ma
    F=μ*Fn


    3. The attempt at a solution
    I have no idea
     
  2. jcsd
  3. Oct 5, 2011 #2
    The point here is that FN= |G| , they have the same numerical value (are equally large), but they go in the oppposite directions , thus one is positive, the other is negative, + or - sign.

    Since gravity pulls downwards, G, the gravitational force points downwards, N is the normal force (in the opposite direction).If we choose positive direction upwards, gravitational pull is negative. with g=-9,81 m/s^2. Remember G=mg, using that FN= |G| will give you the normal force and you can calculate the frictional force from your information.

     
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