# Understanding Pressure

Ok so what happend is that i started to think about Submarines and why exactly you cannot open the bottom heatch at lets say the Challenger Depth. At least i am pretty sure you cannot do that. Right ?

Because, i was told that if you were to open the Heatch, water would still rush in and kill you rather fast. But why ? All of the Water and thus all of the Pressure is above you. Here is a bad illustration of what i mean

My theory: In theory you can do that, but now, since there is no floor, all the Pressure from above is resting on the Water below. Which itself is the support for the Submarine. And with no floor, the Water gets forced in. So what happens is that the Air is acting like the Floor that is missing. But while the Floor could resist the Pressure from above pusing it against the support from below, the Air cannot. And thus gets compressed.

Is that right ?

The problem is that when you are inside the submarine you usually get more or less normal atmospheric pressure (maybe a little higher). What keeps the submarine from imploding is the structural resistance of the hull (usually an inner and an outer hull).

The water pressure at the Challenger deep is in the order of 1100 bar (8 tons per square inch). So if you opened the lid, the volume of the air would be compressed to about 1/1100 of its original volume. Assuming an air volume of 3 m^3, all that would be compressed to roughly 3 liters of air.

But the biggest problem would be that while 70% of our body is water (and incompressible), our lungs and internal cavities are filled with air. So what would happen is that the air inside of our torso would be collapsed to almost 1/1000 of its original volume. Additionally, it would probably be very difficult to breathe (ie. flex our muscles to fill our lungs with air) due to the external load.

# fluid pressure on boundaries

The following is from a lecture on fluid mechanics. It shows the fluid pressure direction on different boundaries.

What should be evident, is that pressure is always normal to the surface boundary. I.e. pressure can act in all directions. So, what happens when a can is submerged in water is the following

You might notice that the pressure above is less than the pressure below (so for a thin object the pressure difference would not be significant). This is how buoyancy force is generated. However, this is not the discussion here, and you can just consider that the pressure is all around equal.

On the other hand, the air inside the submarine is exerting pressure on the walls - again normal to the walls. Its more or less like a balloon in the figure below

So what would happen if you opened the bottom hatch in the submersible y, what happens is that now you have a water air boundary. On one side there is a 1atm air and in the other its about 1000atm pushing the other way.

• Aha this means my general idea of how the Pressure gets appled, i.e The pressure from above forces the water below into the Hull, is right ? thanks for the answer ! Dec 22, 2020 at 21:05
• Since, I am not entirely clear what you mean by the pressure above, I tried to explain a bit more about pressure. Hopefully, alongside with the other posts , it will make sense.
– NMech
Dec 23, 2020 at 6:17

It is incorrect to imagine there is very little pressure being applied to the submarine from beneath it. To a stationary submarine, the pressure it experiences is nearly identical at all points on its surface. The difference in pressure will be the incremental pressure increase due to the depth of the water increasing as you go from the top to the bottom of the submarine (i.e. hydrostatic pressure). This means the pressure beneath the submarine is actually higher than the pressure above the submarine, but the depth change from the top to the bottom of one submarine is such a small number it would not appreciably change the pressure given the surrounding pressure is already on the order of 1000 bar.

The thing to understand about water pressure is that it is always equal in all directions.

Sure, the value of the pressure is defined by how deep underwater you are, but that doesn't mean you'd feel the pressure as a weight over your head. Instead, you feel as much pressure coming from above you as from below, from the sides and every direction in between.

The reason for this is basically what you intuited: the weight of water above makes the water below try to get out of the way by moving to the side, but it can't do that either because the water molecule next to it is also trying to get out of the way.

However, if you open the bottom hatch, a "chain reaction" happens, with the column of water pushing water down, which pushes the water next to the submarine sideways, which pressures the water beneath the submarine to rush in through the hatch until the pressure in the submarine (or at least the areas made available to the water through the hatc) is equal to that of the water.