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Homework Help: Friction on a black on an inclined plane

  1. Nov 26, 2007 #1
    1. The problem statement, all variables and given/known data

    A block of mass 2kg is on an inclined plane and held steady. The coefficient of static friction u between the block and the plane is 0.5. Determine whether or not the block moves when released if the angle the plane makes with the horizontal is
    (a) 20 degrees

    3. The attempt at a solution

    to find n i did this:

    2*10*cos20 = 0.94

    so friction is 0.47

    then i did this:

    2*10*sin20 - 0.47 = 6.37

    so acceleration is 6.37/2 = 3.19m/s^2

    so it looks like the block does move when tilted by 20 degrees...

    but i feel like i got quite a big number for the acceleration, am i doing something wrong? am i supposed to approach this question differently?
    Last edited: Nov 26, 2007
  2. jcsd
  3. Nov 26, 2007 #2


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    Homework Helper

    If the downward force is F and the frictional force is F_R

    and [itex]F=mgsin\theta[/itex] and [itex]F_R=\mu mgcos\theta[/itex]

    consider what happens if [itex]F-F_R<0[/itex] and what happens if [itex]F-F_R>0[/itex]

    one says that there will not me motion and the other says that there will be

    Are you sure that is correct? I mean the frictional force part
    Last edited: Nov 26, 2007
  4. Nov 27, 2007 #3
    sorry made a stupid mistake:

    2*10*cos20 = 0.94

    and i found a more reasonable way of doing it

    the answer is supposed to be negative so the the box doesn't move
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