if a paper a glass half full of water, then the glass is flipped, paper doesn't fall, y??

Question

i know when it's full that the pressure of water acting on the paper is less than that of air from the other side..

but when the glass is half full, air and water pressure act on the paper from inside the glass which makes that pressure > that of air yet it doesn't fall..

i got two answers but i'm not usre:
1 - about water cooling air inside glass so it has low pressure (but i tried it with hot water and worked)
2 - about the size of glass: http://www.newton.dep.anl.gov/askasci/phy99/phy99x72.htm

i don't know if they're right, can any1 tell me and give me more details??

Answer 1

Constant water temp has little or nothing to do with this experiment - you have proved this yourself.

If the air in the glass is not vented, its weight is in the same situation as the water - it can't oppose the atmospheric pressure outside of the glass because it can't push down with any appreciable force since it would necessarily need to create a partial vacuum above it in the glass - and this would only increase the differential pressure that's holding the paper in place - this won't happen.

If the volume of air above the water is cool when inverted - and then slowly heated - I'll let you try it and see what happens when the air is allowed to expand and apply an additional force to the water without creating a partial vacuum in the glass.
Yep, you guessed it - do this experiment over the sink.