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Re: Bernoulli Principle



The problem with molecular arguments is that they involve objects that
can not be seen. Such arguments are really only work well with
"theoretical" thinkers according to Anton Lawson. So this type of
argument may be extremely ineffective in "introductory courses". For
more information go to his papers in JRST.

On the other hand the MPEX survey (Joe Redish) shows that most students
tend to treat derivations as just proofs that it is OK to use the
equations, and they do not use them to connect ideas. This method is
actually not very good either.

John M. Clement
Houston, TX


On 06/25/2003 03:57 PM, Wolfgang Rueckner wrote:

The Bernoulli Principle (or rather equation) is obtained from
conservation of energy applied to the fluid.

Yes, obtained that way, among other ways.

Epstein also makes a
plausible argument (in his Thinking Physics book) as to why the
faster moving fluid has a lower pressure than the slower moving
fluid
from Newton's 2nd law -- imagine being in a tiny submarine moving
along with the slow moving fluid; as you move into the region where
the fluid is moving faster, you are accelerated in that direction
and
therefore there must be a net force on the sub in that direction.

OK.

My question is this -- can one make an argument about what the
pressure difference ought to be from a molecular motion point of
view? And I'm not talking about a detailed kinetic theory of gases
derivation but rather a plausible argument that could be used in an
introductory physics course.

I strongly recommend against this.

It is of course possible in principle ... but doing it correctly is
too
tricky for an introductory course.

Experience has shown that the best approach involves two stages:
-- derive the macroscopic fluid properties in terms of kinetic theory
-- derive Bernoulli's principle in terms of the macroscopic fluid
properties (pressure, velocity, density, et cetera).

Most people who attempt to short-cut this procedure end up mired
in fallacies.
http://www.av8n.com/how/htm/airfoils.html#sec-fluid