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Homework Help: Coefficient of friction problem

  1. Feb 17, 2008 #1
    1. The problem statement, all variables and given/known data
    A 4.00 kg block starts from rest at the top of a 30° incline and slides a distance of 2.10 m down the incline in 1.30 s.

    3. The attempt at a solution
    I solved for the frictional force acting on the block and got 9.64 N.

    I need to solve for the coefficient of friction. I am having some trouble figuring that out. Im not sure how to find the normal force and once i get the normal force do I use the formula force(friction) = mu * normal force

    Any help would be appreciated.
  2. jcsd
  3. Feb 17, 2008 #2
    F(friction)=mu*F(normal), where F(normal) is the force perpendicular to the surface of the plane that the block is sliding on. It comes from the weight of the block itself. Draw a free body diagram of the block to find the normal force, you'll have to use a bit of trig to get it. Good luck :)
  4. Feb 17, 2008 #3
    If i multiply the mass by gravity to find the force of it as if it were not on an incline, and then multiply that by the sin of 30, does that give me the normal force?
  5. Feb 17, 2008 #4
    no it would be cos 30
  6. Feb 17, 2008 #5
    So it would be 9.8*4 = 39.2, but then do you divide that by cos 30, or multiply it?
  7. Feb 17, 2008 #6
    if you draw a diagram of the components of the force you will be able to answer this type of question much more easily. You multiply by cos 30
  8. Feb 17, 2008 #7
    Ok i see that, thank you for your help
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