Re: [Phys-l] question about Bernoulli

[John Denker <jsd@av8n.com>]

On 11/25/2010 09:07 PM, William Robertson wrote:
> Is it possible to understand the  
> Bernoulli effect in terms of molecular motion and changes in molecular  
> motion on a level that makes the effect "obvious" once one pictures  
> what the molecules are doing? Until I get through everything in this  
> thread, I don't yet know the answer to that question. 

Here it is, a week and a half later, and I still don't understand
why that is a question.  I hate to belabor the obvious, but the
obvious answer remains the correct answer:

The fluid slows down when it climbs a pressure gradient.

 -- That is true heuristically and formally.
 -- That is true microscopically and macroscopically.
 -- That is true locally and everywhere.
 -- That is true in every other way.

Also:  If you apply Bernoulli's principle outside its domain of
validity, you will get wrong answers.

> Absolutely. I do a simulation with teachers in which  
> they pretend to be gas molecules with others being their container.  

Maybe that's the problem.  That's not a particularly apt model.
The velocity of the people is determined almost entirely by what
they choose to do with their feet.  Maybe in some fantasy-world
they would choose to accurately model the pressure and velocity 
of a fluid flowing through an orifice ... but in the real world
they won't.  They don't know how.

And as Bob L. observed, even if you have a computer simulation 
where the density, velocity, and pressure are known to be accurate, 
it is not trivial to instrument the simulation so as to make it
easy to comprehend the patterns and laws involved.  It can be done,
but it involves a level of sophistication far removed from a bunch 
of naive people milling around.