Eli5 why hippos can’t swim because of their weight yet blue whales can
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Hippos are not only heavy, they are extremely dense. One blue whale weighs more than one hippo, but the blue whale is relatively larger than it is heavier. Same reason a two-pound rock sinks but a zillion-ton cargo ship floats.
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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).
It is the same reason why a small needle can sink in the water, but big ships cannot. In general, things float in the water, because they have density less than the water itself. This can be managed by how much air you suck in, your body among other things. Hippo do not have that ability, but blue whales do. Hence they are able to swim.
Are we hippo shaming now?
It’s not just about weight or mass, it’s about density and buoyancy. A blue whale can hold 5,000 litres of air in their lungs and this provides a ton of buoyancy. Whales have less bone density as well. And then you add blubber into the mix which is extremely buoyant as well.
So less dense bones, blubber to make it easy to float, and a ton of air helping make then even more buoyant.
There are two reasons! But you should know that hippos do swim, they just don’t dive. They love floating in rivers!
Inside many creatures in the sea is a special bladder. This bladder fills not with water or pee, but with fat or air! Those with air, release it when they need to sink, fill it with air when they need to rise.
You know how when butter is in the fridge, it’s hard but when it warms up in the pain, it becomes a liquid? That hard fat is heavier than the liquid. This is how fat bladders work.
Many whales have this body part, hippos do not.
Hippo skin and bones are made to help them live on land. The skin is tough and the bones are heavy. Water is comfortable because it eases their weight and they have a lot of fat to help them float. However, it doesn’t harden! Their skin and bones are pretty dense, meaning it is not very safe for them to float in deep water. They also can’t hold their breath!
Whales are large but not dense at all for their weight. They have special filters and a way to get rid of the water that they breathe in: spouts! They can blow all the water out!
Hippos can look cute, but they are very dangerous and can attack people. Many people who work with them say they are mean and bad tempered!
When thinking about whether or not something floats, the easiest concept for me was “does the thing weigh more than the water it is replacing?” A whale weighs less and a hippo weighs more. For example, a plank of wood that’s the exact size of a piece of gold would likely float where the gold wouldn’t.
LOL – ya’ll have obviously not seen the videos of hippos swimming. I googled it and wow! They are beasts in the water. After all, they are “water horses”.
I think the confusion here comes from the idea of weight vs density. An aircraft carrier is very heavy too, yet it floats. Hippos live on the land at least some of the time and they are big animals, so they need very strong bones to support their weight. A whale has no such need for heavy dense bones. The end result is the hippo is denser than water and sinks while the whale is less dense or neutral so it can maintain any depth it likes.
That said, it’s a little silly to say they “can’t swim”. If you watch underwater videos you will see them gliding along quite quickly in the water while only occasionally touching the ground. and they shoot up to the surface, and then back down to the bottom. What they can’t do is float.
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