Re: Bernoulli Principle, 2nd try

[cliff parker <cparker@CHARTER.NET>]

I have not kept up with this thread previously so I don't know if my ideas
have been discussed but I'll tell you a bit about the discussions that
happen in my high school class.

CONSERVATION OF ENERGY!  The particles are generally moving randomly in any
fluid.  The kinetic energy of the particles is distributed evenly in all
directions.  Therefore the pressure on any surface (wing, golf ball, etc.)
will be the same on top as on bottom since particles are just as likely to
crash into the top as the bottom.  If conditions arise which cause more
particle to move in one particular direction than another there will be less
kinetic energy available for the particles moving in other directions, in
particular down onto the wing or one side or the other of the golf ball.

When we discuss Bernoulli I always demonstrate by blowing air across the top
of a golf ball with a shop vac.  The ball hovers without any obvious means
of support.  Very cool demo.  One of my student last year described what was
happening molecularly using similar ideas to the ones which I tried to
describe above but stated more simply.  The air stream is blowing the air
particles from around the top of the ball away so that they can not bump
into the ball the way they would have before the shop was turned on.  The
air particles below the ball are unaffected by the shop vac's air stream and
therefore bump into the bottom of the ball just as they normally would.  I
hope this helps.

Cliff Parker

Never express yourself more clearly than you can think.  Niels Bohr
----- Original Message -----
From: "Tucker Hiatt" <thiatt@USFCA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, June 30, 2003 1:36 AM
Subject: Bernoulli Principle, 2nd try


> At 3:57 PM -0400 6/25/03, Wolfgang Rueckner wrote:
> > 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 appreciate your question very much, Wolfgang.  I am disappointed
> that the answer seems, so far, to be "No; no one (on PHYS-L) *can*
> explain Bernoulli's Principle from a molecular point of view."
>
> I certainly wish I could do it.  Many of my introductory students
> enjoy learning about the Bernoulli effect, and then are disappointed
> when I am unable to explain it.  I am able to take a molecular point
> of view in explaining other fluid phenomena (e.g., static pressure,
> buoyancy, capillarity, and Pascal's Principle), but not Bernoulli's
> Principle.
>
> So, forgive me PHYS-Lers, but I must re-issue Wolfgang's question:
> FROM A MOLECULAR POINT OF VIEW, WHY DOES FLUID PRESSURE VARY WITH
> FLUID SPEED?
>
> - Tucker