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I Cone rolling without slipping

  1. Apr 15, 2016 #1
    <<Mentor note: Thread split from https://www.physicsforums.com/threads/torque-opposite-in-direction-to-change-in-angular-momentum.866882/>> [Broken]

    it can be proved that in this case pure rolling without slipping is impossible
    ive assumed the cone to be right circular
    proof by contradiction

    https://drive.google.com/open?id=0B5m-h82V0Ej2b1g5YmFhSUk5UEE
    https://drive.google.com/open?id=0B5m-h82V0Ej2aFRxR2RzRlFmaVk
    https://drive.google.com/open?id=0B5m-h82V0Ej2aGxLRENyYmdXZVU
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Apr 15, 2016 #2
    we do observe but rolling without slipping about axis in post #1(z-axis) is impossible
    here is 1 eg of cone rolling without slipping (without ext force)

    the long line shows path of com
    https://drive.google.com/open?id=0B5m-h82V0Ej2YlVSNlB2UVl2SE0
     
  4. Apr 15, 2016 #3

    Simon Bridge

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    ... please be clearer: you are not really saying that rolling a cone without slipping is impossible and yet it happens?
    Note: diagram meaningless without labels. Shouldn't the cone com move in a circle? That is what happens with every cone I've rolled.

    What about:
    https://www.physicsforums.com/threads/lagrangian-mechanics-cone-rotating-over-a-plane.670424/
    ... in this case, though, the weight-friction couple is not enough to lift the cone off the surface.

    How about:
    https://www.physicsforums.com/threa...-its-side-without-slipping-on-a-plane.750264/
     
  5. Apr 15, 2016 #4
    Lets assume all cones roll without slipping about z-axis thru apex
    Lets now repeat this with cones of decreasing apex angle till it is close to 0
    Object suddenly stops rotating about z-axis and rolls in straight path
    this is a discontinuity in observation
     
  6. Apr 15, 2016 #5

    Simon Bridge

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    No it isn't... we observe cones rolling in circles and we observe cylinders rolling in straight lines.
    Are you, therefore, saying that a cone cannot roll in a circle with it's apex as the center?
     
  7. Apr 15, 2016 #6
    no it cannot except if com is directly above apex (while rolling) (ie cone with a certain obtuse apex angle)
    with no ext force object can rotate only about line thru com
    cone can rotate (with rolling ) but only about line thru com
     
  8. Apr 15, 2016 #7
  9. Apr 15, 2016 #8

    Simon Bridge

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    OK... be that as it may, how does that address the question?... allow a cone executing a circle by rolling without slipping so that the z axis is at the center of the circle. Its just not on the apex. The angular momentum of the cone about its symmetry axis is still changing direction... does this not imply an unbalanced torque?

    Note. In the com frame, the com is stationary: that is what "com frame" means.
     
  10. Apr 16, 2016 #9
    at last i figured it out
    my previous posts may be bit incorrect
    rolling without slipping is always impossiblefor right circular cone
    i had proved it for axis through apex in my previous post
    a simpler proof--
    for rolling without slipping
    velocity of foot of altitude on base (P) must be = velocity of instantaneous point of contact (Q) furthest from apex
    for any axis normal to ground surface , normal dist to P is not equals to normal dist to Q
    so any curved motion is impossible (due to same velocity of 2 points at different distances from same axis)
    translation is also impossible since the instantaneous point of contact just below P will have diff forward velo (due to equal angular speed and diff cross-section radius) (cross-section is normal to symmetric axis not to ground surface)

    what we observe right circular cones rolling is actually cones slipping for small instants
    when right circular cones is given push
    near the apex friction acts backwards (ie cones slips forward near apex)
    near the base friction acts forwards (ie cones slips backward near base)
    this makes cones roll and rotate

    however rolling without slipping is possible for oblique cone whose base is normal to ground surface
    this can visualized by slicing the oblique cone normal to ground surface into very thin rings
    consider only the slice at apex and that at the base
    this can be related with a differential drive system
    slice at apex remains at rest and oblique cones rotates about z-axis through apex
    on static friction is required at apex

    addressing initial ques angular p about z-axis is const . torque about z-axis is 0 , so it is balanced
    angular p about other axis changes. torque about other axis= friction near apex * r , so it is balanced
     
  11. Apr 16, 2016 #10

    Orodruin

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    Please stop linking to google drive documents. Apart from forcing people to leave the site, there is no guarantee that external documents will be maintained.

    Furthermore, your assertion that a cone rolling without slipping is impossible is directly false. The cone is a flat surface and can roll without slipping on a flat surface. This can be easily verified by looking at the area the cone would sweep on the surface, which is going to be diffeomorphic to the cone itself.
     
  12. Apr 16, 2016 #11
    please disprove this
     
  13. Apr 16, 2016 #12

    Orodruin

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    I just did, if you read my reply properly.
     
  14. Apr 16, 2016 #13
    what is wrong with my proof
     
  15. Apr 16, 2016 #14

    Simon Bridge

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    @hackhard
    Perhaps see:
    Jackman H. (2008) Rolling Constraints
    http://www.ingvet.kau.se/juerfuch/kurs/amek/prst/07_roll.pdf
    ... page 8.

    Jackman starts out much as you do but arrives at a different conclusion.
    Working out the constraints for rolling without slipping of a right circular cone on a plane surface is a standard exercise for classical mechanics and engineering students.
     
  16. Apr 16, 2016 #15

    andrewkirk

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    It's easy to see why a cone of any angle can roll without slipping.

    Let the angle between the cone wall and its axis be ##a##. For a point ##P## on the cone that is distance ##r## from the apex, the cone circumference at that point is ##2\pi r\sin a##. Say the cone is rolling at a constant angular velocity of ##\omega## radians per sec relative to its (precessing) axis. Then as point ##P## touches the ground it has linear speed ##\omega r\sin a## relative to the cone's COM. Say the cone is rotating around the origin at rate ##\beta##. THen the speed of that part of the cone relative to the ground is ##\beta r##. So to have no slipping we require the two to be equal, which requires ##\beta r=\omega r\sin a##, that is ##\beta=\omega\sin a##. Since ##r## is not part of this equation, it is possible for it to hold at all radii, so the cone can roll without slipping.
     
    Last edited by a moderator: Apr 16, 2016
  17. Apr 16, 2016 #16

    Simon Bridge

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    It's quite tricky to figure out what you are saying since you are unclear in your definitions.
    But I'll give it a go since the other disproofs provided you do not show you where your own reasoning has failed you.
    I cannot be sure - but I can try to point at places that you can check for yourself.
    ... I think more detail is needed here: it is unclear if you are mixing up reference frames or just assuming the end result. But I suspect this is primarily where you come unglued. In the x-y-z frame in your diagrams, the velocity of Q is zero (points of contact must have zero relative velocity to avoid slipping - hence "assuming the result" when you ruled this out.) Your own diagram shows P and Q having different velocities.

    All the points are also accelerating... I think you a imagining that the cone rolls like a row of vertical disks ... which is not the case.
    A point on the surface of the cone traces out a tilted circle.

    Does this mean the cone cannot roll, without slipping, in a straight line?
     
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