Eli5: How come we don’t feel the millions of pounds of air coming down above us? And why aren’t we so much stronger without it?
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Consider a suction cup. If you press it to a surface, you are clearing the air from between the cup and the surface. Other than a tiny bit of adhesion from the rubber, there are no other forces – the entire reason a suction cup sticks to the surface is that there *isn’t* air pressure on one side but there is on the other. So, the force that you feel when pulling on the section cup – that’s air pressure in action.
If you break the seal, there’s air on both sides, and you can move the suction cup freely. This is what’s happening to our bodies. There’s air on all sides of us, compressing us, but not restricting our motion through the air. 1,000 pounds on one side and 1,000 pounds on the other side equals 0 pounds of net force (but a lot of compression).
That’s why the air pressure doesn’t make us particularly strong. We don’t need to be strong to deal with the air pressure, we just need a biology that can cope with that level of uniform compression. We’re mostly made of water, and water is nearly incompressible, so our bodies can handle pretty huge changes in pressure with no issue – our bodies only encounter mechanical problems when there are uneven forces (pulling or pushing on one part of our body and not another), which creates internal tension and can tear tissue or break bones. The actual problems we encounter at extremely low or high pressures are not these major structural things, but stuff like fluids evaporating into low pressure, gasses being absorbed in the wrong amounts from high pressure, or the few membranes that do maintain some kind of internal pressure not being able to adjust to the extreme value.
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