Re: .Bernoulli and curve balls.

[jmclean@chem.ucsd.edu]

Richard W. Tarara says:
> The obvious solution to all this is to never talk about curve balls, but
> then to take Leigh's advice litterally, I don't think we could talk about
> any 'real world' phenomenon in introductory and/or HS courses at all. 
> After all we don't deal correctly (or realistically) with friction even
> when we do include it in calculations, we almost never deal with air
> resistance, and _what about_ those massless strings, frictionless pulleys
> and the like.  If we don't over-simplify (LIE as Leigh seems to say) at
> some point, we are left with precious little that we can talk about.

I think you are missing a distinction Leigh is trying to make between a
simplification and an incorrect answer which happens to give the right
answer.

Massless strings, frictionless pulleys, and so forth are OK because they do
not change the undderlying physics that is being taught.  Another way to
look at it is that these simplifications can be approached in a limiting
way.  You could do an experiment with a particular weight and a steel
cable, a rope, a string, and fishing line: you would be getting closer and
closer to a massless string.

On the other hand, an explaination can give the correct qualitative answer
but still be dead wrong.  I'm making a distinction here between 'dead wrong'
and 'wrong because it is simplified'.  Take the caloric model.  It gets
some answers right, but it is fundamentally different from the correct
treatment.  One clue to this is that there is no limiting sequence from the
model to reality.

Imagine that the Bernoulli effect does tend to curve all spinning balls in
the 'correct' direction, but that the magnitude of the effect is many
orders of magnitude smaller than the Magnus effect.  That is, say that the
curve is 99.9% due to Magnus and 0.1% due to Bernoulli.  In that case,
saying "the ball curves because of Bernoulli" is not a simplification.  It
is, at best, a misrepresentation.

-- 
--James McLean
jmclean@chem.ucsd.edu
post doc
UCSD