Mystery Corks
Note the cork on the left is foating at the side of the flask while the right cork is floating in the middle - ALWAYS.
Obtain two identical glass containers with smooth circular rims. No pour
spouts. Obtain two identical corks. Fill both containers with water to the
point where the meniscus is ALMOST gone, that is, there is just barely a
curving of the water at the edge. Now, overfill one of the containers so
it acquires a "positive" meniscus, that is, the water level
bulges up over the rim of the container. Now place a cork in each
container. Show the kids and ask for an explanation. They can touch it all
they want. They are amazed that two identical setups yield two opposite
effects. In the "+" one, the cork will always migrate to the
center and in the "-" one it will migrate to the edge. ALWAYS.
Why? Each cork feels an upward buoyant force due to its water displacement
(Archimedes) till it can reach the HIGHEST point. Here it will be in
equilibrium where the buoyant force equals the gravity. Where are the high
points? For the "+" it's in the center and for the "-"
its at the edge. Cool, huh?
