I’ve always wondered and never understood how buoyancy works, especially with huge metal ships that I think should surely sink.
In: Physics
I can’t believe everybody is getting so complicated and still missing the point.
Imagine a glass of water, half filled.
Add a small rock to the glass of water, and the water level rises. The water level rises in a manner directly related to the volume of the rock. For instance, if you could calculate the the volume of the rock to be 5 cubic centimeters, then the water level will have risen the same as if you had simply poured 5ml more water in.* Anyway, the rock sank. But, it had to move that water upward, and in response, the water is exerting an opposing force downward, with a net change equal to the weight of the water that was lifted — that is, the volume of the rock multiplied by the density of the water.
So, how does something float? It floats when there is an equilibrium between the weight of water getting pushed upward, and the weight of the object doing the pushing. The equilibrium is only reached when the object continues to have excess volume as it ‘tries’ to press into the water — that is, it must be overall less dense than water (although not necessarily homogenous in density, for example a boat that is essentially filled with air). This happens in any container of water, even if it is as big as the ocean.
* In fact this is a nice way of imperically determining the volumes of things.
Edit:
So that was the simple version. Here’s something for a six year old, when you’re ready.
You wonder why the rock is pushing down *and* the water that was elevated is also pushing down? You wonder where is the balancing upward push? Well, the water actually is pushing in all directions, upon itself and upon every surface. So beneath the sunken rock water is pushing upward, just as it pushes upward from below anything buoyant. But, since the rock is now below the waterline, water is also pushing downward on it — very nearly negating the upward push.
But it doesn’t exactly negate the upward push. Remember the water pushes upon itself as well. This we call pressure. The pressure of the water may as well be identical to the weight of the water above itself at any given depth. So for example, if you examined a 1″*1″ slice of water that was 128″ below the surface, that slice has roughly 8 lbs total extra pressure than exists at the surface. (Because 1″*1″*128″ is a gallon, and weighs 8 lbs.) Or, as we say, any sized slice of water at 128″ depth has roughly 8 lbs *per* square inch more than that water at the top surface.
So what does that do? It actually compresses the water, increasing its density slightly but measurably at increasing depths. So that as an object pushes through deeper and deeper water, heavier volumes are being lifted. Objects that normally have the same density as water (water near its own surface that is), but which are more resistant to compression than water is, can sink below the surface, and yet find equilibrium before contacting the bottom. This is important to how fish and submarines control their depth, and it relates to those cool thermometers that have labeled weights on glass balls of air, and is exactly how those cheap antifreeze concentration detectors function.
It’s also why a rock slows down as it approaches the bottom of a pool — and why you can’t say that the pressure from below the rock is exactly the same as the pressure from above.
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