Buoyancy

[Kyrisch]

This may seem strange but I never understood the actual mechanism by which things sink and float. For instance, layers of air: Why should cool air sink? I know that it is denser, because that is the reason that is taught in elementary school, but just because it is denser doesn't mean the force of gravity is any stronger upon it. It just was never clear to me.

[antimatter]

It's just something you have to think about for it to make sense,

now that you ask it it makes me question what I've been taught as well, but

if something is packed with more molecules, it 'sinks'. Gravity pulls on it more than

something with a lower density.

 

 

. . .

........

[Kyrisch]

I do think about it. And I can't isolate a specific physical mechanism by which it works, which there must be. It's not like quantum physics or something. I mean, it makes sense that dense things should sink but that's just from daily observation. What I'm looking for is the why.

[antimatter]

I suppose, as Klaynos says, it's time to take out the WP pages.

 

http://en.wikipedia.org/wiki/Buoyancy

 

Density

 

If the weight of an object is less than the weight of the fluid the object would displace if it were fully submerged, then the object has an average density less than the fluid and has a buoyancy greater than its weight. If the fluid has a surface, such as water in a lake or the sea, the object will float at a level where it displaces the same weight of fluid as the weight of the object. If the object is immersed in the fluid, such as a submerged submarine or air in a balloon, it will tend to rise. If the object has exactly the same density as the fluid, then its buoyancy equals its weight. It will tend neither to sink nor float. An object with a higher average density than the fluid has less buoyancy than weight and it will sink. A ship floats because although it is made of steel, which is more dense than water, it encloses a volume of air and the resulting shape has an average density less than that of the water.

[Klaynos]

It's do do with gravity, the force on one element of a dense fluid is greater than that of a less dense fluid so it forces it's way to the bottom.

 

The same logic can be applied to solids in fluids....

[ecoli]

Or, in other words, there's two opposing forces. Gravity pulling downwards, and, by virtue of density, a force pushing upwards. A less dense substance will float to the top of a more dense substance (think of oil floating on water). An object will obtain an "equilibrium" as a result of the opposing force of gravity and the force of buoyancy