Re: [Phys-l] question about Bernoulli

[John Denker <jsd@av8n.com>]

On 11/23/2010 06:31 PM, LaMontagne, Bob wrote:
> However, these arguments unfortunately all boil down to "the
> pressure is lower because it has to be".

That is an utter travesty of what has been said.

It would be more fair to say that if the fluid did not speed up,
there would be a host of consequences.  It is easy to observe
that these consequences do not occur.  It is also easy to rule
them out on firm theoretical grounds.  I'm talking about consequences
such as unbounded accumulation of fluid somewhere in the pipe.

Similarly, if the pressure did not decrease there would be a host 
of consequences.  It is easy to observe that these consequences 
do not occur.  It is also easy to rule them out on firm theoretical 
grounds.  I'm talking about consequences such as gross violations of
the conservation of momentum.

I remark that you could travesty the other laws of physics in the
same way:  You could complain that F = ma "because" F has to equal ma.  
But why would you want to?

On 11/23/2010 06:59 PM, Philip Keller wrote:
> > Like the conservation laws, Bernoulli's eqn seems to have the power
> > to predict what will happen on the global scale while leaving the
> > local scale mysteriious.  I think that is the feeling that got this
> > particular thread started.

I have no idea where this is coming from.

I do not see anything mysterious, locally or globally, in the
Bernoulli equations.  They are a direct consequence of the laws
of motion.

The fluid slows down when it climbs a pressure gradient.  What
is mysterious about that?  I daresay it would be pretty mysterious
if the fluid did /not/ slow down when subjected to an unbalanced
force!

> > our intuition craves an explanation that is "local". 

What is not 'local' about a parcel slowing down as it climbs a
pressure gradient?

> >  I think that is the feeling that got this particular thread started.

This has been going on for a week.  I didn't understand what 
the question was back then, and I still don't.

It seems that some of these questions should be asked on the 
metaphysics list, rather than the physics list.  The correct 
physics answers have been given, but for some reason have not 
been accepted.