I understand that in a tube with water instead of mercury, after filling it of water and put it downwards, it should stay at $10.34$ meters high. I did the experiment myself with a glass of water and a deep plate, and it works(the glass stays full), although I have some doubts:

$1)$ Since atmospheric pressure is constant, then, since the glass was like 10 cm long, and the water can achieve 10 meters, then why doesn't the glass break? Maybe it's too tough?

$2)$ If now, instead of putting the glass full of water into the plate, I just put it empty downwards, then water doesn't go up, contrary to what I expected: Because atmospheric pressure is constant, it should pull the water of the plate down and make it enter the glass, or at least, make it out of the plate. My explanation of this is that the empty glass isn't really empty, since it has air, and this air would pull water down with the same pressure as outside air. Is this reasoning correct? Doesn't it matter that the glass contains almost no air, and there is considerably more air outside(my house)?Why?

3) I put the glass downwards with half of the water in it. It also remains in rest. If I suppose the argument in 2) is true, i.e, that the air inside exercises the same presurre as air outside, then where does the weight of water stay in the equation? Because water should accelerate downwards then.

Edit: I think my second question has more to see with the nature of atmospheric pressure. If its origin is from mass, then I don't get why the water stays at rest. If on the contrary its origin is electromagnetic or smthg, then I can more or less understand why it doesn't matter how much air there is.

  • $\begingroup$ What do you think the pressure is inside the glass (a) when (a) air is inside and (b) when water is inside? $\endgroup$ Nov 5, 2017 at 22:14

1 Answer 1


1, The pressure of the air outside the glass is pressing in with the same force the water is pressing out. If you had the top of the glass in a vacuum and air pressing in from the bottom it would (probably) break

2, Yes - the air inside the glass is at atmospheric pressure - exactly balancing the pressure outside

3, The water can't go down because the weight of all the air outside is pushing it up. Perhaps it is easier to think that if it did go down that would create a lower pressure inside the glass and that would suck the water back up.

Air pressure is entirely about the weight of the air. If you have a problem picturing air as having weight (because it isn't very heavy compared to things around us) imagine the same experiment with two liquids of different density like oil and water

  • $\begingroup$ About 2, I still don't get why it doesn't matter that there is little air inside the glass compared to the house or the world. If I myself put into a box in which I just fit in, would I still feel the same atmospheric pressure? All this based in the fact that atm. pressure is due to air mass. $\endgroup$
    – Diego
    Nov 11, 2017 at 23:12
  • $\begingroup$ Imagine the air in glass was blue tennis balls and the air outside was red tennis balls. The red balls press up with some force until the blue balls are compressed enough to press back with the same force - this force is atmospheric pressure $\endgroup$ Nov 11, 2017 at 23:26

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