When we travel by airplane, the pressure changes. What does that mean exactly? Why does it make my ears hurt?
Equally, deep sea diving and submersibles. Ive read that the glass has to be massively reinforced to stand the pressure. Is the change in pressure the same type for air and sea? I.e does pressure increase for both?
Edit; Everyone did great! I understand now! The answer is “Water be heavy” and “Air be heavy. Less air above you when you fly so less pressure”
In: 13
Pressure is the way fluids confer forces applied to them everywhere they go.
Air (or any gas) is like a spring, if force is applied to one end, it pushes on the other; but if it is pushed on from both sides, it compresses. Without Earth pulling the air down, it would un-spring away into space quite tremendously.
That’s why you can have pressure containers and vacuum chambers: you can stretch or compress that air spring. Releasing it will make air flow in or out. And if the spring is too strong, the container may burst violently.
Meanwhile, water (or any liquid) is more like a rigid metal rod, it almost won’t compress, definitely not in a way you can notice. Pushing against it will do nothing unless something somewhere else gives in.
Thus water in a container is just there (or not). It is not a loaded spring. Yet it will conduct forces through it.
In your typical life, the air and water pressures result from their weight. Hold a large bucket of water up, it takes quite a bit of force to do so. Now imagine that above you, right now, there are dozens of kilometers of air. While air is quite light, it still weighs a bit, roughly a thousandth of the same volume of water. In other words, there are effectively a few meters (~10 to be exact) of water pressing down. That’s a lot more than you can likely lift. Going with the analogue, there is a high stack of springs, and their weight compresses the bottom ones quite a bit. Hence why the atmospheric pressure goes down when you go up.
To put it into numbers, that’s about 10 tons (~2 elephants) per square meter. So several elephants are continuously stepping on you. Luckily, the matter in your body withstands it easily. You can actually have an elephant step onto a thin porcelain plate without breaking as long as the plate is well-supported from below. Solid and liquid things are almost indestructible as long as all forces are fully countered in every direction.
Hence why you usually do not feel the air pressure, it is (almost) the exact same from all directions: you essentially get squished together from everywhere, but you are not a soft foam ball, but a blob of water and bones. Maybe you get smaller by a millionth of your size compared to being in the vacuum of space; you simply won’t notice even if you try to compare (not recommended!).
Your ears hurt, because there is just the slightest bit of difference between the in- and outside. When the airplane rises (or sinks again), the outside pressure changes as we have seen with the springs. But if the various caverns in your head are clogged, usually by mucus, it takes quite some time for the inner pressure to follow suit. Therefore there is now a difference in pressure, maybe a few percent at worst. Still enough to hurt quite a bit on something as fine and thin as an eardrum.
Underwater, the pressure is much higher. As we saw, ~10 meters of water is about all the atmosphere above us. Thus at 10 kilometers depth, we have a thousand times(!) our typical pressure. In other words, a ton per square centimeter, or an elephant per finger.
If you want to have not just one but an entire vertically stacked and huge horde of elephants to be held up by a glass pane, you need one very thick boy of one. Hence why we usually only do small windows surrounded by massive steel on submersibles.
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