From: Paul Doherty (pauld@exploratorium.edu)
Date: Tue Nov 28 2000 - 10:05:47 PST
Message-Id: <l0311070ab649a22e9389@[192.174.2.173]> Date: Tue, 28 Nov 2000 10:05:47 -0800 From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole Bernoulli or not?
Since the last Bernoulli discussion I found a great site.The beginner's
Guide to Aeronautics.
It is sponsored by NASA and its author Tom Benson is an aerodynamicist who
has spent a lot of time teaching teachers about aerodynamics.
He adresses many of the myths such as the "equal transit time theory" your
student noticed.
http://www.lerc.nasa.gov/WWW/K-12/airplane/bga.html
Be aware that the newton explanation F = dp/dt explains the force of lift
on a wing in terms of the downward deflection of the air by the wing. (The
air is given momentum p downward. The wing exerts a force on the air, the
air exerts a reaction force on the wing.)
The Bernoulli theory is an explanation of lift in terms of the pressure
difference between the top and bottom of the wing. The pressure difference
arises due to conservation of energy. (When the air speeds up as it starts
to flow over the top of the wing there must be a pressure difference to
accelerate the air. The pressure in front of the wing is atmospheric
pressure the pressure over the wing must be below atmospheric.)
Both explanations can be used to correctly explain lift.
Since when used correctly F=ma and conservation of energy are powerful mand
correct methods to understand the basic physics of a problem, if you find a
website that claims one of these is totally wrong then you might suspect
the author of using one theory incorrectly.
I use the Newton explanation myself.
Paul Doherty
Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld
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