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Direction of friction force

  1. May 6, 2014 #1
    Which of the following statements are true??

    OPTIONS:

    A)a rotating ball is kept on a rough inclined plane.It may remain at rest till it is rotating.

    B)A rigid body rolls up an inclined plane without sliding. The friction force on it will be upwards.

    C)A rigid body rolls down an inclined plane without sliding. The friction force on it will be upwards.

    D)A rigid body is left from rest and having no angular velocity from the top of a rough inclined plane. It moves down the plane with slipping. The friction force on it will be upwards.......
     
  2. jcsd
  3. May 6, 2014 #2

    Simon Bridge

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    Neat - how are you thinking about these different options?
     
  4. May 6, 2014 #3
    I thought that the first option was wrong as while rotating,,,,... when kept on rough surface due to frictional force it leads to translation
     
  5. May 6, 2014 #4

    Simon Bridge

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    Very good - keep going.
    Don't be afraid to post possible answers that are wrong - we've all been there.
    It's just that I cannot tell how to advise you unless I see how you are thinking.
     
  6. May 6, 2014 #5
    I think there is a typo in part A. right now it says
    "A)a rotating ball is kept on a rough inclined plane.It may remain at rest till it is rotating."
    That doesn't seem to make sense. I think is should say
    " A)a rotating ball is kept on a rough inclined plane.It may remain at rest till it stops rotating."
    in which case I think it's true.
     
  7. May 6, 2014 #6
    Ok then how can u support ur statement ???? Reason???
     
  8. May 6, 2014 #7
    Reason?


    Ok then how can u support your statement??? Reason for being at rest???
     
  9. May 6, 2014 #8

    Doc Al

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    I don't understand what is meant by "is kept". Can you rephrase this option?
     
  10. May 6, 2014 #9
    "Is kept". Means. "Is placed"
     
  11. May 6, 2014 #10
    The friction force uphill might just be enough to cancel the gravity's downhill component might it not? And if it does, won't it remain at rest until it stops spinning?
     
  12. May 6, 2014 #11
    Ohh! I got it.. Thank you
     
  13. May 6, 2014 #12
    Then is the fourth option correct? If so why?
     
  14. May 6, 2014 #13

    Doc Al

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    What do you think and why?
     
  15. May 7, 2014 #14
    You are right .

    But I would like to know is it possible for the sphere to move uphill if friction exceeds the component of gravity ?
     
  16. May 7, 2014 #15

    Simon Bridge

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    Why not try it - get a wheel and see if you can make it roll uphill.
    Or just look and see if cars can drive up hills?
     
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