Eli5 why hippos can’t swim because of their weight yet blue whales can
In: 7
It’s not about weight, it’s about *density*.
The way buoyancy works is that if something is *more dense* than the fluid (air, water, etc.) that it’s in, it will sink. If something is *less dense* than the fluid, then it will float.
Remember that density is mass divided by volume — if you have two items of equal weight, but one is much larger than the other, then the larger one will be *less dense*. Likewise, if you have two objects of the same size, the one that weighs less will be less dense.
So that’s why hot air balloons float — hot air is less dense (weighs less per unit volume) than cool air. So hot air rises. Helium weighs less per unit volume than air, so helium balloons float.
In water, a brick of foam is less dense than water, so it will float. Air is less dense than water, so air bubbles rise in water to the surface and vinyl tubes filled with air float.
Hippos are denser than water so they can walk on the riverbed. Whales, on the other hand, are very close to the density of water so they are *neutrally buoyant*. They can easily surface and sink using mechanical means (that is, propulsion).
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