I agree with kallog.
Maybe I expressed myself unclear: The measured buoyancy is the result of the (downward) force due to the weight of the object and the the resulting force of pressure differences.

And of course I don't deny the connection to the density of the liquid and the object. Since this density is the result of the volume (causing the pressure differences) and the mass in a g-field (causing the downward force).

But it's not the density difference as such causing the buoyancy! That's why I'm always referring to a zero-g environment.
Without gravity the density differences between liquid and object won't change. But there won't be any pressure difference in the liquid or downward force of the object anymore, resulting in a buoyancy force of zero. (I admit: I'm ignoring microgravity, thermal diffusion, surface tension, quantum mechanics and a lot of other aspects since they are not part of the principle in discussion).

Why am I always nagging about this?
Paul designed a device with an elevated water column, which is fine. I guess he will try to use some object (possibly hydrogen) to float along this column and sink again. That's ok.
When we are discussing the energy gained or lost during this process we will have to calculate the work necessary to achieve any change of density, movement and so on at different points.

That's why it is important for us tho have the same understanding which pressure and forces are applying to an object somewhere in the water column.

In my opinion the vacuum (or low-pressure air) at the top of the straw is not exerting any negative force (suction) at the water column below it. In case of a vacuum it just doesn't exert anything (how could it since "it" isn't there).
The water in the column has still its weight and would fall downwards. The air pressure on the open part of the system is not matched by the air pressure on top of the water column.
That's why this pressure is pressing the water into the straw, until the weight of the water in the straw balances the pressure difference between air pressure and the vacuum.

In the water column we have the usual pressure gradient (from 0 to 1 atmosphere) resulting in just the same buoyancy of any object as usual.

If Paul agrees to this explanation, I have to apologies since any misunderstanding could very well be on my part and a possible misuse of the word buoyancy smile


on a side note:
Since water is dielectric we could apply an electric field from the outside on the water column which exerts an upward force on any water molecule. Thus we could compensates the weight of the water (letting it really elevating it). The water column would have no internal pressure gradient, any object unaffected by the electric field would sink. Regardless of its density.

[Edit: found a video of levitating frogs and water in a magnetic field]

Last edited by Momos; 05/02/10 06:50 PM.