Eli5: Underwater pressure


Why do people, submarines, and essentially everything that dives too far into the water get crushed? I get that it’s the weight of the water, but shouldn’t the pressure just come from the top and not everywhere, as if there were a weight on you?

In: 7

Water (and other liquids) transfers pressure in all directions simultaneously and equally. Imagine you have a glass of water, and you put a tightly fitting piston into it and start pushing the water down. Obviously the side walls of the glass will just break at some point, and water will pour out. This is generally not what would happen if there was some solid object in the glass instead of water, that’s why liquids and solids are different. So in case of a submarine the walls of the submarine are the walls of the glass and the weight of all the water above it is you pushing down on the piston.

When you smash something against a table, the smashing is from you pushing down and the table pushing up.

The weight of the water above you is pushing down, and the water below you is pushing up, thus you crunch from both sides.

Edit: and all around. The water is being compressed everywhere so it’s also trying to squeeze into you from the side, if it can.

It’s not weight, it’s pressure. Think of water as really dense air. When the pressure inside the submarine is significantly less than the pressure outside, it compresses. You can replicate this at home by sucking the air out of a plastic bottle. Same principal. One thing submarines do to combat this, is slowly increase the internal air pressure so there is less of a differential. This has to be done slowly, to prevent it causing problems for the humans inside. Likewise when they come up to the surface, they need to undergo a decompression process. Just like when scuba-diving.

It’s a matter of pressure differences I think. Think about how a space ship is pressurized and how the air is trying to get out in every direction. It works in reverse when the pressure is greater outside the object than in. Technically yes it is pushing down on it, but liquids don’t have a ridged structure, so it’s going to try to fill that lower pressure area from all directions.

The universe loves entropy, so every system is being acted upon in order to try and equalize. I don’t have a degree on this topic though so I’m sure someone can explain it better.

It is exactly like having a weight on you. You focus on the weight on your shoulders but you feel it in your feet too, right? Same thing, just water is all around you not just right on top.