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Rolling problems

  1. Feb 24, 2013 #1
    hi everybody I have two questions releted to rolling motion and I want the answer for these questions if you can please

    the first problem:
    when a cylinder rolling in surface which has friction and suddenly the friction gone and the surface become frictionless.
    The question is will the cylinder keep rolling? or will it slide (moving in liner velocity only)?

    Another problem releted to yoyo problems

    If there is yoyo falls from certain height and while it falls there is some kind of rolling without slipping then suddenly the thread of the yoyo is cut and the yoyo falls in frictionless surface

    The question is will the yoyo move while it's in the graund? or will it stay in it's place and just rutate in its place? if your answer is it will rotate in the same place will it rotate forever ?

    I hope you understand my questions and I hope you answer me neerly

    thank you
     
    Last edited: Feb 24, 2013
  2. jcsd
  3. Feb 24, 2013 #2

    Doc Al

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    Well, what do YOU think? Unless you describe your thinking and what confuses you we cannot provide much help.
     
  4. Feb 24, 2013 #3
    ammm
    there are two things I think about it the first one is the cylinder will rotate for a while then it will just slide but the problem here is the newton first low said if there is no external force that means the object will stay as it is (if it roll it will keep rolling)
    but also we know that the only cause of rotation it the friction so if there is no friction
    the segma torqe will be equal to zero so how can it rotate ?
     
  5. Feb 24, 2013 #4
    As I thought you have no answer for this problem
     
  6. Feb 24, 2013 #5

    Doc Al

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    Torque is required to change the rotation. If it's already rotating (as it rolls), then it will remain rotating if there's no friction to slow the rotation.
     
  7. Feb 24, 2013 #6

    Doc Al

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    Impatient, are we?
     
  8. Feb 24, 2013 #7
    so as I understood from you it will keep rolling forever?
    if that was your answer what do you say about the main thing in rotation which is the friction? if it keep rotating that means the friction is not Required for rotation motion ? so we can say that the ring (for example) can rotate or roll in frictionless surface.

    thank you
     
  9. Feb 24, 2013 #8

    Doc Al

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    What would stop it?
    What do you mean?
    If something is already rolling, why would you need friction to maintain it?
     
  10. Feb 24, 2013 #9
    can we prove that in experiment?
     
  11. Feb 24, 2013 #10

    Doc Al

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    Can you remove all friction?
     
  12. Feb 24, 2013 #11
    NASA has a device that works on making the force of gravity is equal to zero ,so we don't have force of gravity and we can make the room empty ( without air ) so we don't have air resistance
    for that we can get segma of force equal to zero:wink:
     
  13. Feb 24, 2013 #12

    Doc Al

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    Really? Or do you must mean 'weightless' as in free fall? Gravity doesn't go away.
     
  14. Feb 24, 2013 #13
    what I mean you can ignore the force of gravity

    and that device can helps you to do that:smile:
     
  15. Feb 24, 2013 #14

    Doc Al

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    For a short while. Not forever.
     
  16. Feb 24, 2013 #15
    That short while is more than enough to prove that:smile:
     
  17. Feb 24, 2013 #16

    haruspex

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    Quite so. Friction is usually involved in establishing rotation. If there is ongoing acceleration, e.g. rolling up or down an inclined plane, friction is likely to play a role in continually adapting the rotation rate so that rolling is maintained. But if a ball is rolling on a flat plane, no losses, there is no angular acceleration, so no continuing need for friction to maintain a rolling state.
     
  18. Feb 25, 2013 #17
    Thank you very much
    I got it but really what make me confused that I heard same story which happened
    to two of my doctors which that when they drove their cars suddnely they drove in black ice (Almost to be frictionless surface) their cars slid and they lost the control of their cars ​
     
    Last edited by a moderator: Feb 25, 2013
  19. Feb 25, 2013 #18

    haruspex

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    Losing control on ice means that you are unable to produce a force when you need one. That includes slowing down, cornering, and maintaining a constant speed up or down a slope. So long as they tried to go at constant speed in a dead straight line on a level surface the tyres would have stayed in rolling contact and they would not have been aware of their loss of control, but that very rarely happens in driving.
     
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