# 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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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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Pressure pushes. The outside of a plane is pushed in by outside pressure, and out by inside pressure.

If outside pressure goes up, then either the inside pressure must go up to match or the walls have to make up the difference.

Usually, it’s some combination of the two.

Regardless, the particles inside don’t know what the outside pressure is, but the walls do because they’re exposed to it, and if the inside pressure is adjusted at all to compensate for the outside pressure, *then* stuff inside feels it. In airplanes, the inside pressure is lower too, but in airliners it is not *as* low as outside pressure.

Pressure compresses your airplane/submarine a little, the material is never 100% rigid. So it gets a tiny bit smaller and the air inside is also compressed a bit.

But the hull catches most of that force, the pressure inside a submarine is much lower than the pressure outside of it.

Equal and opposite reaction.

The particles inside just know they have to provide equal force in the opposite direction as the particles outside.

Math that children are unlikely to understand is heavily involved.

Questions like this are why algebra, trigonometry, comics, matrices, calculus, and physics exist.

Someone actually endeavored to figure it out.

Airliners aren’t pressurized to sea level, it’s usually 5-6,000 feet in the cabin. You still get the sensation of ascending.

Sea level pressurization would take way more energy.

Aircraft are not completely enclosed. The air conditioning system of the aircraft gets air from the engines. That’s why in most airliners your AC vent shuts off momentarily before you are pushed back on to the taxi way and turns on again during engine start. The environmental control system of the aircraft pressurizes the aircraft through conditioned air and regulates the pressure by managing how much air gets vented out.

>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.