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Direction of friction

  1. Feb 9, 2016 #1
    Hi all,
    Can anyone please help me out with the direction of friction in a case where there is a smooth spherical body pure rolling on a rough surface? What is the direction of friction?
    Also, is it possible to have pure rolling without friction?

  2. jcsd
  3. Feb 9, 2016 #2
  4. Feb 9, 2016 #3


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    Depends on the inital situation. Most frequent case: if a ball is sliding over a surface, the backwards friction will make it change over to rolling without slipping.

    But if it hits the ground spinning like crazy (around a horizontal axis), the forward acting friction will accelerate the motion while adjusting the rate of rotation until there is no more slipping.

    And yes: pure rolling without friction is possible. All that is required is ##v = \omega R ## (speed = angular speed times radius)

    [edit] sorry, missed the 'pure' as in pure rolling (meaning rolling without slipping). Agree with Russ #5.
    Last edited: Feb 9, 2016
  5. Feb 9, 2016 #4


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    You can always potential run into trouble if you work with models that are a mixture of ideal and practical. If you have 'no friction' then your model is very limited. You have to work with a very low value of friction - not zero.
    You would need to set up that condition with some external force to produce the rolling.
  6. Feb 9, 2016 #5


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    I think life is simpler: any time an object is rolling without slipping at constant speed, the friction force is zero - regardless of how it got to be that way.

    If you want to quibble about exactness of such a scenario, roll a ball down a shallow incline. At some point it will reach a terminal velocity, where friction against the surface is exactly zero.
  7. Feb 9, 2016 #6


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    You're right . . . . . until you have to deal with yet another question on PF about it. :wink:
  8. Feb 9, 2016 #7


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