Fill a glass half full of water and it will have a concave up
meniscus. Place a cork in the center of the water and it's in unstable
equilibrium. Any perturbation to the side and it continues on up the
meniscus until it touches the side of the glass. Now fill the same
glass to overflowing, so the surface tension creates a concave down
meniscus. Now a cork at the center is in stable equilibrium. Move it
to the side and it returns to the center.
I'm looking for a force explanation for what's going on. There are
legitimate energy arguments, but I really want a forces explanation.
As I see it, there are three possible forces acting on the cork. One
is gravity and the second is the buoyant force. The third is the force
of adhesion between the water molecules and the cork. When the cork is
off center, all of these forces can't cancel. There must be a net
force (toward the edge of the glass in the first case and toward the
center of the water in the second case) acting on the cork.
I have my own explanation (I think the adhesion forces, related to the
surface tension forces in the water, are unequal), but I'm not
completely sure it's correct, so I'd be grateful for any input.