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

[John Denker <jsd@av8n.com>]

I'm all in favor of doing experiments.

Then, having done the experiment, it is important to correctly 
interpret the results.

Consider the three assertions that "Toothpaste is not glue, 
and it's not particularly sticky. ... Therefore, there is no
net tensile force."  The first assertion is literally true 
in some narrow pedantic sense, but it is highly misleading, 
and the second and third assertions are not true in general.  
They are not true of any commercial toothpaste I have ever 
seen.

The fact is, typical toothpaste is sticky, especially in thin 
layers.  It took me two minutes to design and carry out an 
experiment to show an example of this.  I put a thin layer of 
toothpaste on the wide end of a funnel and used that to glue 
the funnel to a plate.  I was able to pick up the system 
using the funnel as a handle, thereby demonstrating that the 
toothpaste has significant tensile strength and stickiness.

The funnel remains open at the small end, so there is no
possibility of any suction-cup effects.  The plate is entirely
supported by tension in the sticky paste.

In fact, typical toothpaste is a semi-solid.  It is not a very
_good_ glue, but it does exhibit thixotropy, which is one of 
the properties one expects a good glue to have.

===========

More generally, I am reminded of one of Murphy's Laws, namely
the one that says "If everything seems to be going smoothly,
you have obviously overlooked something."

We should all keep in mind that this is a complicated 
system.  Maybe there is a simple way of looking at it that 
explains everything, but I very much doubt it.  So far, 
everyone who has claimed a simple explanation has been 
overlooking big parts of the problem.

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.

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.  A suction cup is a tricky thing, but of all 
the things it depends on, the presence or absence of a 
monolayer is not one of them.

One important part of the physics starts with the Poisson 
ratio, which I hinted at earlier but which has not heretofore 
been explicitly mentioned in this thread.

An incompressible liquid will have a Poisson ratio of 0.5 exactly.  
An immobile H2O monolayer and/or an immobile glue layer will 
have a lower Poisson ratio.  Similarly, the effect of an O-ring 
(supported by an O-ring groove) is to drastically change the 
effective Poisson ratio of the system as a whole.

This is a complicated multi-dimensional system.  Discussing it 
without reference to the Poisson ratio is going to give a very 
incomplete picture.  (Indeed, even if we include the Poisson 
ratio, we still don't have a decent description of everything 
that is likely to happen, although including it is a step in 
the right direction.)  In particular, a discussion that focuses
on only one dimension (e.g. tension versus uniaxial compression)
is only one part of the proverbial elephant.  The physics very
much depends on what is going on in the other dimensions, starting
with whether the glue / paste / putty / O-ring is free to creep
sideways.

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.

Therefore, seeing a strain gauge in compression wouldn't prove
anything to me.  Equivalently, seeing a strain gauge in tension
wouldn't prove anything either.

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

A good rule of thumb:  Fluids are tricky.

We spend our lives surrounded by fluids, and there are a lot
of people who think they "intuitively" understand fluids.
Encyclopedias and textbooks (from first grade on up) are full
of "intuitive" explanations for how fluids behave, including
how a wing produces lift.  They're virtually all wrong;  very
significantly wrong.

Recently this group has been fussing over a statics problem.  
Imagine how much more complicated fluid /dynamics/ is.