Re: [Phys-l] buoyancy on a submerged pole

[brian whatcott <betwys1@sbcglobal.net>]

On 11/3/2010 8:23 PM, Chuck Britton wrote:
> At 9:07 PM -0400 11/3/10, Ken Caviness wrote:
> > It occurs to me that the glue between the bottom of the box and the
> > bottom of the aquarium might be treated as a viscous fluid.  Even if
> > the glue does seal the box to the aquarium so that no water can seep
> > between them, as the aquarium is filled with water the pressure at
> > the bottom of the aquarium rises, and this pressure might then be
> > expected to squish the glue around the edges, in turn raising the
> > internal pressure inside the thin layer of glue itself, and being
> > communicated to the bottom of the box, despite our best efforts to
> > prevent this from happening.  If this were to occur, the box would
> > effectively again have a buoyancy force.
> >
> > Of course, if you insist on "ideal glue", like massless strings and
> > perfectly rigid rods, ....  :-)
> 	I'm trying to keep it real here.
>
> > Easy to do the experiment:  just put a hook in the top of the box
> > (or pole, as some have said) and tie it to a force sensor overhead,
> > arranging things so there is some tension in the string (which is
> > recorded by the force sensor).  Now add water.  If the glue is
> > communicating a buoyancy force to the box/pole, the tension in the
> > string should decrease.
> I would expect it to decrease if the water vessel has an unyielding bottom.
> For an aquarium - the bottom will sag, But it will sag less if there
> is an empty box glued to the bottom.
> ________________________________________
Yesterday I voted in the wrong side, today I must write more carefully:

In a water container, if a weighty box is added, the weight on the base 
of the water container increases.
If a weighty box is sealed right at its edges to the base, in a way only 
physicists can manage, the down force under the box decreases and the 
base not under the box sees more weight due to the increased water 
column there.

Brian W