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Fictional Vacuum Globe Rises?

  1. Feb 14, 2007 #1
    I've done my own head in - all because I didn't study basic gas physics hard enough. So...

    Assume I have an indestructible, impermeable, rigid globe (of a fictional metal, say) containing a complete vacuum, and I am able to change the size of the sphere at will. My globe and I are standing at sea level. If I keep increasing the size - and so, volume - of the globe, will the globe eventually rise upwards into the atmosphere?

    It will, right?

    I guess the fact that the pressure inside the globe is always zero is kind of throwing me, for some reason.
  2. jcsd
  3. Feb 14, 2007 #2
    Yes it will.
    If your assumptions hold, when you increase the size of the globe, it will finally rise upwards into the sky. Because the total weight of the globe is unchanged, and the Archimedes increases when the volume of the globe increases, so you will get to the point that the archimedes force exceeds the gravity of the globe. Pressure inside the globe has nothing to do in this case.
  4. Feb 14, 2007 #3
    Right. Good. Thank you very much.

    It's a buoyancy thang! I completely understand now. Sometimes all you need to solve a problem is the right word to put in the wikipedia search. In this case, "Archimedes".

    Again, thanks.
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