Re: [Phys-l] Definition of upthrust or buoyancy

[John Denker <jsd@av8n.com>]

On 10/21/2010 12:53 PM, LaMontagne, Bob wrote:

> Yes, the flotation balloons are under compression, but the skin of
> the balloons are under tension (or they wouldn't be inflated). If you
> make a horizontal cross section through the balloon, the vertical
> portions of the skin above and below that cross section are under
> tension. The piling is just a whole bunch of those vertical skins
> layered together around the long axis.

That's another interesting case.

Note the contrast:
 -- The balloons are under compression from the ambient fluid.
 -- They are pressurized in the opposite direction from the
   internal fluid.

Consider a "piling" made of a cylindrical balloon of non-stretchy 
but flexible material.  We are working in seawater.  Depending on
whether we fill the balloon with fresh water or extra-salty water,
we can make it have positive or negative buoyancy.  In both cases
we inflate the balloon so that the skin is under tension.  The more 
we inflate it, the more skin-tension we get.  It seems that the 
lion's share of the tension is due to the internal pressurization,
not due to the buoyancy.

So we have identified three cases that need to be considered:
 -- solid object resting loosely on something else
 -- solid object making a suction-cup seal against something else
 -- deformable object with pressurized internal fluid

These are all simple enough ... provided we are careful not to
confuse one with another.