I found this problem on Brilliant and it's quite interesting because you have 2 tubes that are connected and on both ends they are put under a different pressure:
On the upper hole there is less pressure than on the lower hole, which I then thought, since water moves from lower pressure to higher pressure then it must flow in this tube from up to down, but Brilliant says it doesn't move at all! How is that possible if it exists a difference in pressure?
First of all, water flows from higher pressure to lower pressure, not the other way around. The reason it won't flow (in either direction, even if you start with the tube filled with water) is that the pressure difference, which pushes water from bottom to top, is cancelled by gravity, which pulls water from top to bottom. The reason these forces cancel exactly is that the pressure difference is itself caused by gravity. Pressure lower in the fluid builds up until it exactly cancels the gravitational force pulling down on the liquid above it.
