The water coming out of a hole in the side of a can near the bottom
comes out quickly because the water pressure near the hole is bigger
than the air pressure outside of the hole, and so there is a net force
on a little bit of water that's passing through the hole.
The pressure of the water near the hole is the sum of the air
pressure on the top surface of the water and the extra you get from the
weight of the water. This pressure from the weight of the water is
proportional to the depth from the surface of the water.
For most cans, if they are open, the air pressure on the top
surface of the water is nearly the same as the air pressure outside the
bottom near the hole. So the net pressure pushing the water out comes
from the weight of the water in the can.
If the can is closed and you can push on the air inside with a
piston, you can increase the pressure on the top surface of the water
beyond what the air pressure is at the hole, and the water will shoot
out faster -- you'll have made a squirt gun. Alternately, by pulling on
the piston, you can reduce the air pressure on top so that the air
pressure outside the can balances the pressure from the weight of the
water plus the smaller pressure from the air on top. Water may dibble
out of the hole. If you reduce the pressure on top further, you will
suck air into the can through the hole and it will bubble up to the top
of the can. If the hole is big, water may still splash out even in this
case. If the hole is small, air can go in without water coming out.
(republished on 08/02/06)