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Static Friction of hollow sphere rolling up incline

  1. Apr 1, 2012 #1
    For hollow sphere rolling up incline:

    I know that the kinetic friction will equal 2/3 acceleration since:

    μgcosθ=2/3a

    acceleration = 3/5gsinθ

    so...kinetic friction = 2/5gsinθ

    But how I do calculate the force of static friction??
     
  2. jcsd
  3. Apr 1, 2012 #2
    Would the static friction be the max static friction?
     
  4. Apr 1, 2012 #3
    Max static friction = tan θ times mgcosθ

    Would the static friction exerted by incline on the hollow sphere be the max static friction?
     
  5. Apr 1, 2012 #4

    tiny-tim

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    hi sweetpete28! :smile:
    yes, the acceleration is 3/5gsinθ,

    but there's no kinetic friction for a rolling object (beacuse there's no relative motion at the point of contact) …

    that 3/5gsinθ is the static friction! :biggrin:
     
  6. Apr 1, 2012 #5
    Hi tiny-tim,

    So the magnitude of the acceleration is equal to the force of the static friction??
     
  7. Apr 1, 2012 #6

    tiny-tim

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    oops!

    oops! :redface: i copied the wrong line! :rolleyes:

    i meant that 2/5gsinθ :smile:
     
  8. Apr 1, 2012 #7
    But that is what I did and the answer is wrong:

    For a I got 3.77 --> (3/5)gsin θ = (3/5)(9.81)(sin39.8degrees) = 3.77 (this is right)

    Force of static friction = (2/5)gsinθ = (2/5)(9.81)(sin39.8degrees) = 2.51 (this is wrong)
     
  9. Apr 1, 2012 #8

    tiny-tim

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    what about the mass?
     
  10. Apr 1, 2012 #9
    Mass of hollow sphere = 6.24kg but how does this factor in if:

    μgcosθ = (2/3)a

    solve for a: a = (3/5)gsinθ = 3.77 m/s^2

    so Force of friction should = (2/3)(3.77) = 2.51 N....still dont understand why this is wrong...
     
  11. Apr 1, 2012 #10

    tiny-tim

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    that's an acceleration!

    multiply by the mass :wink:
     
  12. Apr 1, 2012 #11
    ok...

    so Force of static friction = (m)(2/5)gsinθ = (6.24kg)(2/5)(9.81m/s^2)(sin39.8) = 15.7N...??

    Right...?
     
  13. Apr 1, 2012 #12

    tiny-tim

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    should be :smile:
     
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