Eli5 : if pressure is due to the size of the column of fluid above, why is it not lighter inside ?

Question

I know that if you are inside of a hermetic room, the pressure is the same. But I can’t understand *why*. For example, if you enclose water from the bottom of the ocean in a box, shouldn’t it be suddenly at the same pressure than outside, due to the fact there is not the weight of the water column above ?

Thanks!

In: 2

Answer 1

Firstly there are two types of fluids, incompressible and compressible.

For all extents and purposes, water is incompressible. Whereas air is compressible.

For incompressible fluids, you are right in saying that pressure is from the hight of water above you, as it is pushing down and into you. The water isn’t higher pressure, it is just *under* higher pressure. So when you bring it to the surface, it will be the same.

For compressible fluids like air, when the air is under higher pressure, it becomes more compressed and thus higher pressure. So if you brought it to a less pressurised area, it would rush out.

Edit: I wondered for a while about your exact idea. I thought that if you had a roof over you, you shouldn’t feel the same pressure, but you do. It took me a few years of doing physics in school to understand it is the air next to you pushing you. Whereas underwater it is the water above you pushing you.