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

When you close the box, you lock in the pressure. If the box is rigid it’s stuck there. If it’s flexible then the box is just transmitting pressure through the wall. Either way the pressure doesn’t change.

If you take a rigid box, close it at the surface, and sink it then the pressure inside *doesn’t* come up. We call that a submarine.

If you close a rigid box at depth and then extract some fluid from inside the pressure *will* fall, until you’ve got a vacuum in the box or the box fails.