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Imagine a container filled with highly pressurized air. Now imagine that there is some object, maybe a marble, inside the container. Is it possible to cause the marble to "float" within mid-air, due to the forces of the pressurized air acting on it in all directions?

Additionally, why is it that objects tend to spread to the edges of the container, rather than the center?

I imagine this would also depend on the shape, size, and pressure of the container, as well as the shape and size of the objects inside.

OP's MS Paint effort.  From http://i.imgur.com/4H5qjI6.png as posted in comments.

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  • $\begingroup$ Air table (for pucks) are commercially available. Is this what you have in mind? $\endgroup$ – ZeroTheHero Aug 22 '17 at 1:33
  • $\begingroup$ @ZeroTheHero Nope, not like an air hockey table. I made a beautiful ms paint illustration for you: imgur.com/a/b3UkA. The blue thing is the marble, floating within a glass cube that is filled with pressurized air. It is in the center of the cube, not touching any edges. There is no air blowing on it, it is simply stationary air. $\endgroup$ – Mitch Talmadge Aug 22 '17 at 1:49
  • $\begingroup$ Not all containers have a flat bottom, so objects may fall to the edges. $\endgroup$ – EKons Aug 22 '17 at 9:14
  • $\begingroup$ Re, "...pressurized air acting on it in all directions..." That should tell you something right there. The pressure pushing down on the top of the marble will be the same as the pressure pushing up on the bottom. Increasing the pressure of the fluid (air) in the container will not make the marble float. Increasing the density ...(see answers, below) $\endgroup$ – Solomon Slow Aug 22 '17 at 13:09
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Yes.

In practice it probably depends on the temperature, but let's assume not real air, but an ideal gas. Then, all you have to do is to increase the pressure enough for it to have the same (or infinitesimally higher) density than the marble: at this point buoyancy equals the weight of the marble and it can "float".

That will not depend on the shape or size of the object inside, only on its density.

As for the spread you describe, unless there's some sort of flow in the container, there has to be some effective attraction to the wall.

  • Edit: it's important to note that, if the key concepts are not only pressure and flotation, but also air, than, as many pointed out, air cannot be made as dense as a typical marble without changing phase (and or melting the marble).
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    $\begingroup$ Thanks for the info, this is really interesting to me! I would have accepted both answers if I could. $\endgroup$ – Mitch Talmadge Aug 22 '17 at 2:37
  • $\begingroup$ @stafusa but how do you make air dense enough for that without it changing state. You might be able to do it with Radon - as a liquid it is about twice as dense as glass, but I don't know how dense you can make it before it liquifies. $\endgroup$ – Baldrickk Aug 22 '17 at 7:03
  • $\begingroup$ If you make a gas as dense as a marble, I think it's going to feel more like a liquid in most respects, even if by some definition it's not technically a liquid. $\endgroup$ – rjmunro Aug 22 '17 at 10:15
  • $\begingroup$ @Baldrickk (and others), you guys are right, of course, but I hope you did notice that I said "but lets assume not real air, but an ideal gas". Ideal gases don't condensate, because there's exactly zero attraction between its molecules in all conditions. $\endgroup$ – stafusa Aug 22 '17 at 10:44
  • $\begingroup$ @stafusa for some reason I read "ideal" and replaced it in my head with "noble" $\endgroup$ – Baldrickk Aug 23 '17 at 7:50
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Just generally, the density of the "fluid" in which the marble's floating has to be greater than the density of the marble. But air liquifies first, see https://en.wikipedia.org/wiki/Triple_point I don't know if there's any high-enough temperature where air wouldn't liquify before reaching a typical marble's density.

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    $\begingroup$ ... and not melt the marble $\endgroup$ – rjmunro Aug 22 '17 at 10:12
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See the other answers about increasing pressure until the density of the gas is equivalent to that of the object.

Additionally, why is it that objects tend to spread to the edges of the container, rather than the center?

That's just entropy.

The number of states (e.g., position) of the marble that would count as "near the edges" is vastly, vastly larger than the amount of states that we would consider "at the center". Now, divide the volume into an arbitrarily fine grid of voxels (like 3D-pixels) and label each one either "near the edge" or "near the center" by whatever definition you please. You will have hugely more voxels labeled "near the edge".

Unless you actively stabilize the marble in the center, any wayward motion of the marble will put it at a more or less random other place. If we assume that nothing influences it after getting a little nudge in a random direction with a random amount of energy, then it will bounce around like a 3D billiard ball and eventually come to rest somewhere after some of the energy is bled off through collisions with the box (large effect); and the rest by the constant bombardment of gas molecules (minuscule effect, but it is still there).

Simply counting the voxels, it is much more likely that it will come to rest in one that is labeled "near the edge" because there simply are more than those around than the few "in the center" voxels.

(N.B. to address a technicality mentioned in the comments: the above is slightly simplified; in practice you would need 3 categories ("close to the edge", "at the center", "in between"). If the box gets large, a large amount of those voxels would be "in between". Still, the amount "at the center" would be basically constant and very small, while both other categories grow. So you see why it is at least highly improbable that wherever the marble goes, it will not be at the center, or at any other specific place.)

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  • $\begingroup$ I think that's incorrect. For a non-fractal shaped container, the volume close to the edges is vanishing small (for increasing container size or, equivalently, decreasing marble size). $\endgroup$ – stafusa Aug 22 '17 at 11:08
  • $\begingroup$ @stafusa: technically, yes... see edit. $\endgroup$ – AnoE Aug 22 '17 at 11:12

protected by Qmechanic Aug 22 '17 at 11:18

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