Take a glass that is partially filled with water and drop a cork into it. The cork won't float in the center, but keeps drifting to one side and attaching itself to the side of the glass.

\-------------------/
 \                 /
  \##-------------/
   \             /
    \           /
     \         /
      \_______/

Without swirling the water, how can you make the cork float in the center?

(In reply to spoiler by JLo)

I'm not sure about JLo's solution 2.  If the surface of the water is not an equipotential surface (for non-rotating water this means it's not horizontal), a cork at a higher position has more potential energy than at a lower position.  The difference in energy is proportional to the height difference and the mass of the cork.  The amount of water the cork displaces has the opposite difference in potential energy, because it is "missing" whereever the cork is and weighs the same.  This would seem to cancel out.

I think the reason why the cork floats to the side is the surface tension of the water between cork and the side of the glass.  The surface of the water between cork and sidewall can be minimised if the cork is near the side.
 (Otherwise the water surface curves upwards both at the sidewall and the cork, assuming it wets both, which makes it slightly larger.)  So I think the solution is to add dishwashing liquid to the water, which lowers the surface tension and diminshes this effect.

Posted by vswitchs on 2006-10-03 10:16:54