Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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



Comments interspersed


At 10:55 PM -0700 11/4/10, John Denker wrote:
I'm all in favor of doing experiments.

hear, hear - and in keeping with doable experiments - I'd like to withdraw the ill-chosen term 'monolayer'. Any suitable achievable thickness of said water layer will suffice.
Whatever is required to keep the system and the results 'simple'.

(Realizing full well that the ANALYSIS will NEVER be simple ;-) )

On 11/03/2010 06:42 PM, Chuck Britton wrote:
If we let a monolayer of water seep under the box (but keep it tied
down with a thread) we can then call the upward force Buoyancy. Same
amount of force - different name. OK.

Using a "monolayer" to draw a sharp boundary here is a
bad idea. It is a distinction without a difference. The
physics doesn't care whether there is a monolayer there
or not. One monolayer of H2O adsorbed on a surface will
normally act more like a solid than a liquid. Another
reason why it is a distinction without a difference is
that operationally it doesn't matter whether there is a
fluid between the buoyant object and the scale that it
sits on. The scale reads the same, whether there is half
a monolayer, or one monolayer, or five layers, or a thousand
layers. Conservation of momentum requires it.

ok


In any case, we should be more interested in the physics
than in the terminology. One thing that greatly changes
the physics is whether or not any suction-cup effect is
occurring.

My hypothesis (which I am QUITE ready to have disproved - experimentally) is that SuperGlue or threads will exert the SAME upward force on the bottom sheet of glass.
The results might shed some light on the complexities of suction cups but I'd prefer to keep the smoke and mirrors to a minimum.


On 11/04/2010 02:36 PM, Chuck Britton wrote:
So your conclusion is that strain gauges attached appropriately will
show compression as the pole is submerged,

I'm not from Missouri - but I DO look forward to some experimental results.

This was discussed last week. I thought we established that
depending on details, one could obtain either answer, either
compression or tension. A slightly-porous object might well
start out in compression but cross over to tension as water
gradually leaked in.

Again - I'm only concerned with non-porous, lightweight objects.
Leave the complexities for later analysis.

KISS - Keep It Simple, Students !!


=============================

A good rule of thumb: Fluids are tricky.

hence my use of the term 'magic'.