Why does a balloon get squished when you dive deep in a pool?
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Why does a balloon get squished when you dive deep in a pool?
The short answer
Deep water squishes you because of the weight of the water stacked directly above you, not because of how much water surrounds you. The deeper you go, the taller that column of water, so the harder it presses from every side.
How it works
Water is heavy, and a tall stack of it weighs a lot. At any spot underwater, the weight of all the water above is pressing down, and that pressure pushes equally in every direction. Go one step deeper and you add more water on top, so the squeeze gets stronger. This pressure depends only on how deep you are (plus the water's heaviness and gravity), which is why both your ears and a sealed air balloon get crushed more the farther down they go.
What people get wrong
Many people assume the squeeze comes from the total amount of water around you, so a giant lake would crush harder than a narrow tube. It does not. At the same depth, a skinny column of water squeezes exactly as hard as a huge wide lake, because only the height of water stacked above a point sets the pressure. Grown-ups call this surprise the hydrostatic paradox.
The catch
Depth always wins: every bit deeper piles on more squeeze, and the only way to lower it is to come back up, which is why deep divers must move slowly and protect their ears and lungs. The flip side is that width gives you no safety. A small pool is not gentler than the ocean. At the same depth, the squeeze is the same.
Questions kids ask
Does a bigger pool squeeze you harder than a small one?
No. At the same depth the squeeze is the same, whether you are in a tiny tube or a giant lake. Only the height of water stacked above you sets the pressure, not how wide the water is.
Why do my ears hurt when I swim to the bottom of a pool?
The deeper you go, the more water-weight is stacked above your head, and that pressure pushes in on your eardrums from the outside. Coming up a little or gently popping your ears balances the pressure and eases the ache.
Why does an air balloon shrink underwater but a rock does not?
The balloon is full of squishable air, so the rising pressure presses the air into a smaller space. A rock is already packed solid, so there is almost nothing for the pressure to squeeze.
How much does the squeeze grow as you go deeper?
In fresh water it grows steadily with depth. About every 10 meters down adds roughly the same amount of pressure as the entire atmosphere of air above the surface, so deep water adds up fast.
For grown-ups
Hydrostatic pressure in a still fluid is P = rho * g * h, depending only on depth, fluid density, and gravity, and is independent of container width or total volume (the hydrostatic paradox). In fresh water, roughly every 10 meters of depth adds about one atmosphere (~101 kPa). Pressure at a point acts equally in all directions, so a sealed air balloon shrinks evenly as it descends; the increased pressure compresses the trapped gas, with P times V staying about constant (Boyle's law).