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Lump disk mass to a point

  1. Jun 9, 2010 #1

    imagine a disk rotating by an axis that passes through the center of the disk

    how can we find the analogue of a mass rotating by an axis?

    I mean, how can we lump the whole mass of the wheel to a point?

    how much will be the distance of this point from the axis?

    for example to rotate a disk with radius 10 and mass 100 you need the same force as to rotate a point of mass 100 and rotation radius = ??

    I suppose the radius will be some less than 10 (since not all parts of the disk are at this radius), but how much exactly?


    PS: we need to do this in order to calculate other things, eg angular acceleration, etc
  2. jcsd
  3. Jun 10, 2010 #2


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    hello mather! :smile:
    We can't lump it to a point (because it needs to have the same centre of mass as the disc, otherwise it will wobble :redface:), but we can lump it to a ring, with the same mass as the disc, and whose radius is the radius of gyration

    see http://en.wikipedia.org/wiki/Radius_of_gyration" [Broken] :wink:
    Last edited by a moderator: May 4, 2017
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