# Hydrostatic and atmospheric pressure

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1. Jul 4, 2015

### 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 :)

2. Jul 4, 2015

### 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.

3. Jul 4, 2015

### Thom_Silva

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

4. Jul 4, 2015

### Dr. Courtney

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

5. Jul 4, 2015

### 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.

6. Jul 4, 2015

### 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?

7. Jul 4, 2015

### Staff: Mentor

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

8. Jul 4, 2015

### Thom_Silva

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 ?

9. Jul 4, 2015

### Staff: Mentor

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

10. Jul 7, 2015

### Shreyas Samudra

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 !!

11. Jul 7, 2015

### 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.

12. Jul 7, 2015

### Shreyas Samudra

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

13. Jul 8, 2015

### Thom_Silva

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

14. Jul 9, 2015

### Shreyas Samudra

i am unable to open the video link

15. Jul 9, 2015