ELI5, How does air pressure affect things inside an airplane or submarine if they are completely enclosed? how can the air particles inside the shell know what the pressure is outside?
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>how can the air particles inside the shell know what the pressure is outside
This is a misunderstanding, and I don’t think the answers so far are picking up on what’s really happening.
You are right, the particles don’t “know” anything, and **it is only the pressure INSIDE the shell that affects things inside. The air pressure inside an airplane or sub is not kept at sea-level atmospheric pressure.**
Inside a flying plane, the air is still much lower pressure than ground level (just not *as* low pressure as the air outside the plane). If they pressurized the cabin up to full atmospheric pressure, that would create a bigger pressure difference between inside and outside, so the hull would have to be stronger, thicker, and heavier, aka WAY more expensive. So they pressurize it only partially. Enough that people inside are fine, but it’s still much lower pressure inside during flight than when the plane’s on the ground.
Submarines are the same idea but in reverse. They are air-pressurized to a higher pressure than ground level (just not *as* high as the pressure outside). If they were only pressurized to normal atmospheric pressure, that would create a bigger pressure difference between inside and outside, again requiring more hull strength and thickness. So they pressurize it higher, still at a suitable level for humans, but much higher than regular ground level pressure. That pressure pushes outward against the water compression, taking some load off the shell itself.
TLDR: They don’t know. The air pressure INSIDE an airplane is lower than normal air pressure, and the air pressure INSIDE a sub is higher than normal air pressure. Just not *as* low/high as what’s outside the shell. But all the pressure effects result from the changing pressure INSIDE the shell.
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