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Rotation of a sphere-how can it happen?

  1. Mar 2, 2009 #1
    Rotation of a sphere--how can it happen?

    The definition of a perfect sphere would seem to make it impossible to cause it to rotate by the application of any mechanical force.
    And if it ever did rotate or roll, what would make it stop?

    Friction and the perfect sphere are contradictions.

    Basic physics texts seem to have no problem when showing spheres rolling across a surface.
    But assuming enough surface irregularity to account for friction creates a much more complex model.
  2. jcsd
  3. Mar 2, 2009 #2


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    Re: Rotation of a sphere--how can it happen?

    Friction does not require roughness, but it does require atoms. So if a perfect sphere to you means something indivisible (or infinitely divisible), then it might not be subject to friction, but this sphere would also be too "perfect" to possess explainable mass, opacity, thermal conductivity, etc.

    Physics is about abstracting Nature into models. We want to make things as simple as possible, but no simpler (did Einstein say this?). So we allow our sphere model to contain atoms to prevent us from staying up at night worrying about rotational acceleration. Electrostatic attraction between atoms enables friction.
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