Re: .Bernoulli and curve balls.

["Richard W. Tarara" <rtarara@saintmarys.edu>]

Well I'm sure I don't have a very good understanding of Bernoulli--pretty
obvious isn't it ;-( 
but when presenting the equation I've always looked at it as a Conservation
of Energy/unit volume of the fluid where the P term comes from Work/Volume,
that is Fdelta(x)/Vol, rho g h being the grav PE/vol and the .5 rho v^2 the
KE/vol.   So I guess I'm not exactly sure why the ball can't do work on the
air--especially when in a simplified presentation (such as
Miller's--College Physics 6th edition, pg315) it would seem that positive
work is done on one side of the ball and negative work on the other to
yield net work = 0.  

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.

Rick

----------
> From: David Dockstader <DRDOCK00@UKCC.UKY.EDU>
>
> The question to ask about curve balls, airplanes, and shower curtains is
not if
> Bernoulli's law makes things go the right way, but if the conditions to
which
> Bernoulli's law apply exist for these events.  I submit that they do not.
 For
> the curve ball the spinning ball does work on the air so energy within
the
> fluid is not conserved.  For airplanes the engines do work, in a glider
gravity
> does work, again energy in the fluid is not conserved.  For the shower
curtain
> falling water does work on the air and there is no need or continuity on
> opposite sides of the curtain.