Why do plastic bottles “suck” themselves in when the plane is descending?
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If they were closed at altitude the air inside is at lower pressure than the air at the ground so the bottle will collapse on descent. Planes at altitude aren’t pressurized to ground level
When you are standing by the ocean, you have the weight of the whole atmosphere over your head. This can be measured as air pressure. The higher you go the less air there is over your head and thus the less pressure. If you close your water bottle up high and then you descend into the atmosphere, the pressure of the air outside the bottle increases while the pressure of the air air trapped inside the water bottle stays the same. If the bottle was open the higher pressure air would just flow into the water bottle and become equal but it can’t. Instead the realtively weak plastic bottle crumples under the weight to make the pressure of the air outside the bottle and the air inside the bottle the same. The pressure crushes the bottle until either the walls of the container can resist further compression or the pressure of the air inside and outside the container become equal.
Think of putting a balloon in the vacuum hose. When the vacuum is off the pressure around the balloon reduces, so the balloon is round.
When you turn the vacuum on the balloon changes shape, like a water bottle on an airplane, because the air pressure around the balloon has increased.
They’re getting crushed, actually. Believe it or not, the air that’s all around us has weight. As you get closer to the ground, there’s more air so it weighs more. But since you can’t add more air to a sealed bottle, the extra air weight outside the bottle crushes it.
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