Why water in cup suspends when placed upside down in water?

[henry wang]

Why water in the cup doesn't leak out until the opening of the up reaches the surface of water?
Is it because the weight of the water in the cup tries to create vacuum on top of the cup, which exert negative pressure on the water?

 

[BvU]

No. Because 10 km of atmosphere press down on the liquid outside the cup. Read all about it under Toricelli (who did the experiment with a tube and mercury and discovered that 10 km of atmosphere weighed as much as 76 cm of mercury). So before your cup actually shows a vacuum, it would have to be over 10 m high .

 

[CWatters]

+1

Is it because the weight of the water in the cup tries to create vacuum on top of the cup, which exert negative pressure on the water?

A vacuum would be zero pressure. There is no such thing as a negative absolute pressure (eg less than a vacuum).

 

[henry wang]

Thank you!

 

[Keith_McClary]

So before your cup actually shows a vacuum, it would have to be over 10 m high .

You could do this with a (preferably transparent) garden hose in a 4 storey stairwell. Have you seen this described anywhere? I guess the water would boil as you pulled the hose above the 10 m level.

 

[BvU]

Google water barometer.
Boiling is a big word for what happens at the 10 m level, but you are right: the apparent vacuum at the top is in fact very low pressure water vapour (0.023 Bar at 20 C) so a very small amount of water has to evaporate