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Homework Help: Object on an incline plane

  1. Sep 28, 2010 #1
    1. The problem statement, all variables and g
    object on an incline plane has a mass of 10 kgs and the coefficient of static friction is 0.35 at what angle does the object begin to slide.

    2. Relevant equations
    Fnet=ma Ff=sin(theta)(mg)


    3. The attempt at a solution I dont come close to the solution because i feel as though some piece of information is missing. Therefore i cant begin to solve the problem.
     
  2. jcsd
  3. Sep 28, 2010 #2

    Doc Al

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    What's the maximum value of static friction for a given angle?
     
  4. Sep 28, 2010 #3
    isnt it Fn(Us) or normal force times static friction?
     
  5. Sep 28, 2010 #4
    not actuallu u that symbol that looks like a u the micro symbol
     
  6. Sep 28, 2010 #5
    so the normal force is equal to (mg) or in this case (10kg)*(9.86) oh ok i think ive got it now thank you very much...
     
  7. Sep 28, 2010 #6

    Doc Al

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    The maximum value for static friction equals μFn.

    Careful! The normal force does not equal mg. (It would if the surface were horizontal, but it's not.) Hint: Find the component of the weight perpendicular to the surface.
     
  8. Sep 28, 2010 #7
    cos(theta)*mg
     
  9. Sep 28, 2010 #8

    Doc Al

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    Good!

    So what's the maximum static friction force?
     
  10. Sep 28, 2010 #9
    (.35)*cos(theta)*mg
     
  11. Sep 28, 2010 #10

    Doc Al

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    Good.

    So what's the condition for the object to just barely start to slide?
     
  12. Sep 28, 2010 #11
    is this correct???
     
  13. Sep 28, 2010 #12
    sin(theta)mg be greater than (.35)*cos(theta)*mg
     
  14. Sep 28, 2010 #13

    Doc Al

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    Yes. But to solve for the point where the object is just about to slide, set them equal. That will be the critical angle.
     
  15. Sep 28, 2010 #14
    ok i got tan(theta)=.35 inverse tan(.35)= 19.3 deg
     
  16. Sep 28, 2010 #15

    Doc Al

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    Sounds good to me.
     
  17. Sep 28, 2010 #16
    awsome thank you soooo much!
     
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