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If you see my horrible reference picture, you will see that the U shaped tube is placed inside the field where gravitational force exist. And the bottom of the tube is opened, tube is filled with liquid. the density of liquid is ρ, acceleration of gravity is g, and each column's height is h

I used to conclude that the water won't flow down, because the pressure occurred from each side of water column would eventually cancel each other, thus no more pressure exist.

But if that should be the correct answer shouldn't there be a premise that " there exist attraction force between liquid molecules which is greater than the pressure exerted on the end of the liquid (=ρgh)"?

enter image description here

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    $\begingroup$ I have a hard time figuring out what you mean. In which direction does gravity point? $\endgroup$ – Davide Dal Bosco Jul 6 at 17:27
  • $\begingroup$ @DavideDalBosco Downward. sorry I forgot that. $\endgroup$ – WienAudience Jul 6 at 17:54
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    $\begingroup$ It would help if you would provide a moderately detailed explanation of the drawing. $\endgroup$ – David White Jul 6 at 18:07
  • $\begingroup$ @DavidWhite I edited my question let me know if more information is required. OMG, after re-read my question, I found that it's so poorly described. Maybe it's because I was dozing. $\endgroup$ – WienAudience Jul 6 at 18:29
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The answer is somewhere inbetween. Its that the force of the attraction of the liquid molecules to eachother combined with the difference between the air pressures on both ends is greater than the force of gravity pulling down on the water. (of course, the difference in air pressures is zero in this case, but I wanted to mention it because one of your viewpoints mentions pressure).

And of course, there's a few fun details:

  • Surface tension is an important factor in this structure. You want to use a small tube so that the surface tension makes it unfavorable to let a bubble push up past the water. Large tubes will empty themselves rather quickly because they will let air in, wrecking all the nice easy balances.
  • The arms have to be equal in length. As designed this is unstable. If one arm reaches just a little longer than the other, you will have more gravitational forces on one side than the other, and it will pull the water out of the tube. Basically this is how a siphon works. (Except typically we embed one or both ends of the siphon tube in a large body of water, so that rather than sucking air into the other side, you suck more water in)
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