Le Chaud Lapin wrote in
oups.com:

On Oct 16, 3:31 pm, Thomas wrote:
On 16 Oct, 19:41, Jim Logajan wrote:





Thomas wrote:
You may want to check out my web pages
http://www.physicsmyths.org.uk/bernoulli.htmand
http://www.physicsmyths.org.uk/drag.htmfor a closer examination of
the physics behind the aerodynamic lift and drag.

You might want to actually _include_ Bernoulli's theorem somewhere
in your pages. You talk about Bernoulli's equation, Bernoulli's
principle, and Bernoulli's law. And yet none of them are actually
presented. Are you saying they all the same or all different? Why
not use the terminology used by the professionals and stick with
"Bernoulli's theorem"? How about including references to relevant
texts on your pages? It's not like serious texts and lab
experiments haven't been done on the subject for a zillion years.
It helps to show you know what you're talking about by showing
you've first read the professional literature on the subject and
done your own relevant research.

You might also want to redraw your figures so they include vertical
labeled arrows. Then present the assumptions and math needed to
show your work and why you think the vertical magnitudes sum to
zero. Just saying they do, or they only yield a torque, isn't good
enough. It is more useful to _show_ - not pontificate and
hand-wave.

P.S. Chapter section 40-3 in volume 2 of Feynman's Lectures on
Physics is as good a place as any to start.

Bernoulli's theorem is not a fundamental physical law and thus not
required to understand the principle behind the aerodynamic lift. And
its misinterpretation and misapplication quite evidently leads to
incorrect physical conclusions, like the claim that a moving gas
would inherently have a lower static pressure than a stationary one.
The net flow velocity of a gas has per se nothing to do with the
static pressure.

I so agree. The amout of hand-waving that goes on when (presumably
technically-inclined) individuals invoke Bernoulli is perplexing.
Oddly, my college physics book is almost as guilty - after chapters
and chapters of Newtonian mechanics that are quite clear, they seem to
imply just that.

As a thought experiment, consider a large tank containing gas with a
pipe attached to it which leads into a vacuum space. Assume first
this pipe is closed at the end; then the flow velocity in the pipe is
zero because the molecules heading outwards will be reflected at the
end and reverse their velocity (assume for simplicity that the
molecules do not collide with each other but only with the walls of
the pipe and the tank). If one now opens the pipe, the only thing
that changes is that the molecules heading outwards will not be
reflected anymore at the end but simply carry on heading into the
vacuum space (with the corresponding loss of molecules being replaced
from the large tank). So we now have a net flow velocity within the
pipe without that either the density nor the speed of the molecules
has changed in any way. This means that the pressure exerted on the
inside wall of the pipe is unchanged despite the fact that we now
have a net flow velocity within it. So Bernoulli's theorem would
quite evidently give a wrong result here.

Hmmm...technically, someone could argue that, in the vicinity of the
exit hole of the tank, there would be resulting decrease in pressure,
which would be true.

The misapplication, I think, results from too much hand-waving and not
being very specific about what pressure decreases over what. A venturi
apparutus, for example, very clearly demonstrates a drop in pressure,
and that drop is real, but the points chosen to measure the pressure
in the apparutus is very specific.


Hey, i'm waving my hand!

Well, just one finger, to be precise..


Bertie