Hydrostatic and atmospheric pressure

[Thom_Silva]

So I've been wondering, 10.3 meter of water amount for one atmosphere, and according to Pascal's law pressure can compute by P=P0+ ρgh. If we have a glass with 10 cm of water inside , the pressure in the glass would be lower than atmospheric pressure and therefore when you turn the cup upside down water shouldn't fall, like when you have a straw! But in reality water does fall, what am i missing ?

thank you for your help :)

 

[Dr. Courtney]

The pressure under any depth of water is the atmospheric pressure (P0) above the water plus the rho g h from the static water pressure.

 

[Thom_Silva]

I expect that i have a vacuum in the bottom of the glass, so P0=0.

 

[Dr. Courtney]

I expect that i have a vacuum in the bottom of the glass, so P0=0.

P0 is the pressure at the top, not at the bottom.

 

[Thom_Silva]

I'm talking when you turn the glass upside down... pressure at bottom should be 0, right? pressure at the surface should be equal to 1 atmosphere.

 

[Thom_Silva]

My question could be formulated in a different way. If we can hold 10.3 meters of water in a straw, why doesn't the same happen with the glass?

 

[russ_watters]

Water is a liquid and can't hold its shape well enough to stay in. Try the same with a can of jello.

 

[Thom_Silva]

Water is a liquid and can't hold its shape well enough to stay in. Try the same with a can of jello.

So that is the reason...Thank you very much!. In the straw water holds because it's somehow easy for water to maintain it's structure, right ?

 

[russ_watters]

So that is the reason...Thank you very much!. In the straw water holds because it's somehow easy for water to maintain it's structure, right ?

Yes. For the small diameter of the straw, the surface tension of the water is enough to hold it's shape.

 

[person_random_normal]

firstly inverting the glass you have p atm at the open surface of the glass trying to push the liquid inwards BUT
oh man you are forgetting mg the weight of water , the pressure cannot balance that !

 

[Thom_Silva]

No, you're wrong. Pressure does balance that. That's why if you put a card on top of the glass and turn it down, the card would stuck onto the cup as well as the water . Pressure is a manifestation of the weight of a substance on other. I'm pretty sure I'm right, nevertheless it will be good if someone could back me up.

 

[person_random_normal]

hey
the card is so light !
however we all know that water falls off the glass, so then how can you reason that ?
if i am wrong

 

[Thom_Silva]

I could be explaining in words, but this video does a better job :

 

[person_random_normal]

i am unable to open the video link

 

[person_random_normal]

please send the URL

 

[Thom_Silva]

please send the URL

Go to youtube search for : The history of the barometer (and how it works) - Asaf Bar-Yosef