Re: [Phys-l] Bernoulli's Principle

["Robert Cohen" <Robert.Cohen@po-box.esu.edu>]

Rick Swanson wrote:

> 2.  If something interacts with the air and goes up, some air must go
> down.  It's a conservation of momentum thing.

What do you mean by "some air"?  If "some air" goes down and "some air"
goes up, such that the air, on the whole, is at the same level, does
that counter conservation of momentum?

One thing that bothers me about the "air must be projected downward" is
illustrated when an airplane flies horizontally.  What is the difference
between an airplane flying horizontally and simply taxiing horizontally
on the runway?  Is mass being projected downward in each case?  And what
if the plane was "flying" in water?  Is the water projected downward,
and what would that look like to someone observing the water surface?

I think we need to keep in mind that the Earth is there.  And, as John
Denker so elegantly pointed out, air is a fluid.

Air exerts a force when there is a difference in pressure within the
fluid.  For certain situations, a difference in pressure is *consistent*
with differences in speed or a circulation.  However, in my mind, that
does mean that differences in speed or circulations *cause* the pressure
difference or can be used as "explanations" for the pressure difference.
This is like saying that, due to a=F/m, an acceleration *causes* or
*explains* the force.  All it means is that one is consistent with the
other, right?

Suppose you hold a wood board out the car window while driving down the
road.  If the board is oriented with its plane oriented vertically, what
"causes" the pressure on the leading side to be greater than the
pressure on the trailing side?  Is your explanation of this much
different when the plane of the board is at an angle?  If so, why?  This
is where I am confused.

I think this was covered before but I can't find it in the archives...

----------------------------------------------------------
Robert A. Cohen, Department of Physics, East Stroudsburg University 
570.422.3428   rcohen@po-box.esu.edu    http://www.esu.edu/~bbq