Re: .Bernoulli and curve balls.

[Marlyn Jakub <mjakub@physics.otago.ac.nz>]

At 05:13 PM 9/30/96 -0700, Leigh wrote:

> > The important point here is that a curve ball curves not because of
> > the Bernoulli effect but because of the Magnus effect. Drag is
> > important, as is turbulence.
>
> Well, there it is. We fully agree on this point. The curve ball is not
> a manifestation of the Bernoulli effect. I agree that it is properly
> ascribed to the Magnus effect, but in that form it amounts to nothing
> more than a factoid - there is no teachable physics for the 99% behind
> that observation!
Sorry Leigh but I disagree with you...sounds too much like a copout solution.
Perhaps I tend to think of Bernoulli's Principle as a more general
description than Bernoulli's equation. Magnus effect clearly is not friction
free, so Bernoulli's equation does not apply, but the B.Principle applies if
one accepts that the circulation flow around airfoils and cylinders slows
the airflow on the side where the larger side-moving pressure exists.
Similarly, B.Principle states that the other side, where the circulation
direction adds to the unmodified flow field, will thus have lower pressure. 

Something as involved with everyday world as curving spheres should be part
of the world of physics at the first-year level. I disagree with Leigh  on
relegating the topic solely to advanced aerodynamics. Hecht has been
breaking new textbook territory for this topic by including the essence of
circulation and vortices into his text, and I feel grateful for that step.
Sure, the fluid flowfield is not streamline flow, but the model can be
easily modeled by circulation flow superimposed with the steady flowfield. 

Smooth spinning cylinders and spheres do initiate circulation and this seems
to have a lateral forcing consequence which is consistent until nonsymmetric
surface defects are introduced, as in cricket balls and in baseballs with seams.
Proper analysis of the latter objects are advanced topics, but I vote for
developing the curving smooth cylinders and spheres as part of the world I
can explain to first-year students. My two cents.