Re: [Phys-l] Fixed paradigms (WAS: buoyancy on a submerged pole)

[brian whatcott <betwys1@sbcglobal.net>]

On 11/5/2010 12:30 PM, John Clement wrote:
> No you are not missing anything.  A simple free body diagram says it all.
> When something is firmly on the bottom with no water under it, there is a
> large pressure force on the top, and a counterbalancing normal force exerted
> by the aquarium bottom by NTN2 (no acceleration).  So by NTN3 there the
> object is pushing down on the aquarium bottom equal to the force of the
> aquarium pushing up.  At that point you can put glue or other inelastic
> materials between the object and the aquarium as long as you don't allow
> water to seep in.  The interesting cases where wrangling is fun is when you
> have elastic or porous glue.
/snip/
> John M. Clement
> Houston, TX
Hmmm..."A simple fee body diagram. "     That's 'simple' as in 'obvious' 
I expect?    :-)

Then there is this curious "pressure force". That's a new one to me.
The essence of sealing joints against leakage is applying sufficient 
pressure to the
 adjoining surfaces. It was something of a miracle moment, when the 
idea of sealing
 containers against high fluid pressure, by the use of  round 
cross-section o-rings
was developed. The miracle was that the o-ring needed no great 
externally applied
 mechanical compression: an inherent  pressure multiplier effect provides
 reliable seals from that geometry.
 In the case of our kitchen sink experiments it is much the same:
gather the force applied by the water head on a loose container,
and apply it on a thin edge for preference: the several  inches of water 
pressure
is opposed by a considerably higher mechanical  pressure at the foot of 
the container.

Interesting to read the assertion that water pressure normal to seal 
pressure
is ineffective: it comes as news to me that water pressure is in fact 
unidirectional,
not something I associate with pressure, certainly!   (But I probably 
contributed
 to this little issue, by talking of a downward seal  pressure versus 
an inward
water pressure: mea culpa.)

...Continuing the general merriment.

Brian W