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Swing mass overhead - forces involved

  1. Sep 8, 2010 #1
    I was considering the simple scenario: A mass is attached to a string, which is then twirled around by a scientist overhead. Assuming the scientist is spinning the mass fast enough, it will eventually be rotating in plane above the scientist.

    I'm having some trouble working out the dynamics of this system. Obviously the mass is acted on by gravity, and in order to 'rise' from it's initial position will need a force opposing gravity.

    I've reasoned that the tension on the string, while the system is gaining angular momentum from the scientist, will have a component upwards and, provided an appropriate tension, should be enough to get the mass up.

    My problem comes when the string eventually is rotating in the plane parallel to the ground; obviously, there is no component of the string tension to oppose gravity. What keeps the mass from falling in this situation?

    Best guess is that the mass does indeed fall downward, giving rise to an upward component of the force from the tension in the string, which then returns the mass back to the parallel plane, and oscillates like this while there is still tension in the string.

    I considered conservation of angular momentum (neglecting the scientist and frictional forces) but I can't figure how this would stop the mass from falling due to gravity; only that it would have the rotational speed increase as the mass is pulled downward.
     
  2. jcsd
  3. Sep 8, 2010 #2

    Borek

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    Staff: Mentor

    No, it will be not plane, but surface of the cone. Plane is a limiting case for infinite rotation speed.
     
  4. Sep 8, 2010 #3

    Doc Al

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    The mass will rotate in a plane, but that plane will always be below the support point. As you yourself have pointed out, you need a component of string tension acting vertically to counteract the force of gravity.

    As Borek points out, the string will never become parallel to the ground.
     
  5. Sep 8, 2010 #4

    Borek

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    Oops, mass is in a plane, string is on the cone surface, lack of precision on my side.
     
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