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Suppose you have a glass filled with water. Now you insert a straw in that glass.

  1. Is the air pressure inside the straw greater than the atmospheric pressure on the water outside it?(I think it is greater, as the bottom of the straw is deeper than the water level of the glass, so the weight of the air above it is greater. And also because the straw has a smaller surface area. Please confirm it)

  2. If my after mentioned assumption is correct, then the atmospheric pressure in the straw is higher than that of the outside. So why does water get into the straw then(until both water levels become equal)?Why isn't the force exerted on the end of the straw(by this higher atmospheric pressure inside the straw) enough to prevent water from entering it?

  3. And finally does water entering the straw from the glass have anything to do with pressure at all? Or does it only depend on the Force exerted? Because then it makes sense. Even though the pressure inside the straw is greater than the outside,(My after mentioned assumption) I don't think the overall force exerted on the water outside the straw by the atmosphere is less than the force exerted by the atmospheric pressure inside the straw. So that's why it pushes the water up the straw. Is this correct?


1 Answer 1


TL;DR: It is pressure, not just atmospheric pressure.

You are right that at the bottom of the straw there is more depth of air over the surface of the water and more atmospheric pressure.

But at that depth, without straw the pressure is the surface atmospheric pressure plus the pressure of the water above it. Since water is considerably more heavy than air, that is a lot more than the atmospheric pressure at the bottom of the straw, and water would rise through the straw (*).

Related to #3, the difference makes little sense. Pressure is force divided by surface. Since here the surface is constant, pressure and force are directly proportional (if there is 2x pressure then there is 2x force).

(*) Note that, in fact, water did not rise. When you sink an open straw in the water, you just displace the water occupied by the walls of the straw; the water that happened to be in the inside of the straw just remained there. But even if you had inserted in with some form of cap, the moment you removed it the pressure would have made water to rise.

  • $\begingroup$ But there is no water above the straw. When you insert it into the glass the whole straw is empty.I'm sorry, but I didn’t understand your answer $\endgroup$
    – ACRafi
    Commented Oct 11, 2021 at 7:49
  • $\begingroup$ The water just under to bottom of the straw feels a pressure X in all directions, except up where it is pressed with a pressure Y. Since Y < X, the water "goes" up (although it does not go up because it was never removed in the first place, but it would go up even if it had been removed). $\endgroup$
    – SJuan76
    Commented Oct 11, 2021 at 8:06
  • 1
    $\begingroup$ @AbdullahChowdhuryRafi - "When you insert it into the glass the whole straw is empty" - no it's not (unless you keep the other end closed). If the upper end is open, then the air doesn't go down with the straw - as soon as the straw goes in, there's water in it, at the same level as the rest of the glass. Unless the straw is very thin so that there's capillary action. $\endgroup$ Commented Oct 11, 2021 at 8:31
  • $\begingroup$ But why does it stop entering the straw when the water levels are equal then? $\endgroup$
    – ACRafi
    Commented Oct 11, 2021 at 11:00
  • $\begingroup$ Again, it does not "stop entering the straw" because it was not removed in the first place, and it does not move because, if you leave both ends of the straw open, the pressure at the same height is the same in all of the liquid. $\endgroup$
    – SJuan76
    Commented Oct 11, 2021 at 12:37

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