Essentially my question is this: if I get a straw and put it in water, I cover my thumb on the top and pull out, water doesn’t want to leave. Idk why or how but regardless.
How come this same thing doesn’t apply for a swimming pool? Like a body of water is suspended because it has a lid or roof?
In: Physics
Air pressure and surface tension.
Surface tension helps to keep the water surface together, so it doesn’t bend or distort much on its own. Without surface tension, part of the water could start flowing out. Surface tension mostly acts to provide a surface for the other factor here, air pressure.
Air pressure pushes (effectively) equally on all surfaces of an object: top, bottom, and sides. If you block up the top of the straw, that means that the only place that can feel the atmospheric pressure is the small hole at the bottom of the straw. That pressure, in the right circumstances, can be greater than the weight of the water pushing down.
As a result, you can only make this work if:
1. You don’t have too much water to hold up (because the weight is too great and will overcome surface tension + air pressure)
2. The hole the water can fall through isn’t very big
3. The water is mostly stationary (no internal flow/currents)
4. You close up all holes that aren’t pointing more or less straight down.
It doesn’t work with swimming pool sized things because the water is too heavy, breaking point 1.
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