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Mass on an Inclined Plane

  1. Sep 16, 2007 #1
    1. The problem statement, all variables and given/known data
    A block weighing 73.7 N rests on a plane inclined at 24.3° to the horizontal. The coefficient of the static and kinetic frictions are 0.28 and 0.11 respectively.

    A.) What is the minimum magnitude of the force F, parallel to the plane, that will prevent the block from slipping?

    B.) What is the minimum magnitude of F that will start the block moving up the plane?

    C.) What is the magnitude of F is required to move the block up the plane at constant velocity?


    2. Relevant equations

    w = 73.7
    theta = 24.3
    Mus = 0.28 (Don't know how to make that little symbol lol)
    Muk = 0.11



    3. The attempt at a solution
    A.) F = w sin theta - Mus*wcos theta = 11.52 N

    B.) F = fs + wx
    Fmin = Mus*wcos theta + wsin theta = 49.14 N

    C.) F = Wx + fk
    fk = Muk*N = 7.3889
    Wx = fs + F = 11.775 + 11.52
    fs = Mus*wcos theta


    Part C I don't think is right but I don't know what I am doing wrong?
     
  2. jcsd
  3. Sep 16, 2007 #2
    What does it mean for something to be moving at a constant velocity?
     
  4. Sep 16, 2007 #3
    It means that it is not accelerating...Newtons first law I think
     
  5. Sep 16, 2007 #4
  6. Sep 16, 2007 #5
    Oh ok newtons second law.
    So why isn't it...
    F = Wx + fk
     
  7. Sep 16, 2007 #6
    What do you mean by F? So, you understand that the forces must sum to zero because there is no acceleration? Also, since velocity is kinetic, you need to use kinetic friction.

    -Wx - fk + F = 0

    I think you were using static friction. Did you actually get the question wrong, or do you just think you are wrong? It looks like you are right.
     
    Last edited: Sep 16, 2007
  8. Sep 16, 2007 #7
    the force. Yea I understand that it must = to 0.
    so
    Wx = fs + F
    fk = Muk x W cos theta
    ?
     
  9. Sep 16, 2007 #8
    Looks fine to me. Watch your signs though.

    F = fk + Wx

    Actually I am sort of confused now that I look that over once more. Wx = mgsinØ, why do you think it is fs + F? What did you do there?
     
    Last edited: Sep 16, 2007
  10. Sep 16, 2007 #9
    Nevermind I was looking at the wrong thing I think for Wx, but I did try to do that...
    F= (Muk x W cos theta) + (mg sin theta) = 49.14 N
    but it says its wrong
     
  11. Sep 16, 2007 #10
    I get a different answer, make sure you are in degrees and all that.
     
  12. Sep 17, 2007 #11
    F = (0.28 x 73.7 cos 24.3) + (73.7 sin 24.3) = 49.14
    its in degrees
     
  13. Sep 17, 2007 #12
    Kinetic friction coefficient is .11 :)
     
  14. Sep 17, 2007 #13
    omg lol i make the dumbest mistakes!!!!
     
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