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Warm air rising vs centripetal force

  1. Jul 30, 2011 #1
    I assume this is the correct forum for this question... :P

    This is the idea: we have a hollow object (probably a sphere or cylinder), attached to a tether, rotating around a central body. The dimensions and speed I don't have specifics for and shouldn't matter: the point is, centripetal force is equivalent to Earth gravity, so water is pressed against the 'outer' edge of the object.

    Now, this shape is large enough for it to support its own microclimate; i.e. water at the bottom evaporates, rises, and falls as rain. When the object is set on this tether, it is heated from the 'outer' edge of the object. So this is my question:

    Will centripetal force cause water vapour in the object to rise towards the centre of rotation, or outwards (and return to the water)?

    As usual, many thanks to any answers that can be given!
  2. jcsd
  3. Jul 31, 2011 #2


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    At the same temperature, water vapor (molecular weight 18) is less dense than air (nitrogen 28, oxygen 32, CO2 44).

    So because of the buoyancy effect the water vapor will tend to "rise" to the center of the rotating hollow object, against the local direction of "gravity".

    Incidentally, you can show the same effect with a rotating container full of water. Objects that float in water (i.e. are less dense than the water) will tend to move to the center not the outside edge.

    The same principle affects fires in rotating structures (for example fires in jet engines caused by fuel getting into the wrong place, or oil overheating). The fire builds up in the middle of the rotating volume, not around the edges.
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