Finger on the top of the straw, keeping the liquid in ...

lundyjb

So i realize that by putting your thumb over the top of the straw, your eliminating the air pressure from the top. But why doesn't the liquid still fall out the bottom of the straw from gravity, the liquid being more dense than the air below it.

phinds

Hint: do you think it would work if the "straw" were 10 feet in diameter and of proportionate length and dunked into and then lifted out of Lake Erie via helicopter after the top had been closed off?

Chestermiller

With your finger covering the top of the straw, if the fluid were to move downward even the slightest amount, a vacuum would form between your finger and the top of the liquid. Actually, within this tiny amount of space, the pressure would be equal to the equilibrium vapor pressure of liquid, which is very low at room temperature. So the pressure on the bottom of the liquid column would be atmospheric, and the pressure on the top of the liquid column would be essentially zero. This difference in pressure would be sufficient to support the weight of a fairly large column of liquid. In reality, no space forms, although the pressure of the fluid on the bottom of your finger will be less than atmospheric. You should feel suction on the bottom of your finger.