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Re: MATH PHYSICS



Yes of course: I was attempting to teach Granma to suck eggs again:
I prefer to say of Stokes, its an approximate model in the low Reynolds'
limit where in the aerodynamicists' version, Cd is rising as diameter is
falling in the usual Fdrag prop Cd.V^2 model.
This helps to avoid the Millikan syndrome.
(Reliance on physical "laws" instead of models.)
But did I mention, I tend to be heterodox?

Thanks

Brian W

At 01:14 PM 11/27/01, Don Polvani wrote:
Brian W wrote:

>>Not much air resistance in an oil bath, I would have thought, Jack.

>>Could it be that it makes the sums work a little better?

In fact the drag force on a "slowly moving" sphere is proportional to
velocity and not the square of velocity. This applies for motions in which
the Reynolds number (i.e. 2xradiusxvelocity/kinematic viscosity)is small.
For a sphere of one mm radius, moving in water, the velocity must be less
than 0.2 cm/s. See, for example, L.M. Milne-Thomson, "Theoretical
Hydrodynamics", fifth edition, 1968, pp 681,682.

Don Polvani
-----Original Message-----
From: Brian Whatcott [mailto:inet@INTELLISYS.NET]
Sent: Tuesday, November 27, 2001 1:46 PM
To: PHYS-L@lists.nau.edu
Subject: Re: MATH PHYSICS



Brian W

At 09:15 AM 11/27/01, you wrote:
> You're missing low velocity viscous drag, as used in analyzing the
>Millikan oil-drop experiment. See chapter 12 of "The Mechanical Universe"
>(standard edition).
> Regards,
> Jack
>
>On Mon, 26 Nov 2001, Brian Whatcott wrote:
>
> > >David Abineri wrote:
> > > >
> > > > If one assumes that a projectile encounters an air resistance
> > > > proportional to velocity, one can write a differential equation like
> > > > mr''=-mgj - kr' which can be solved for r using an integrating
factor
> > > > e^(kt/m).
> > > >
> > > > The final solution for r, however, does not admit an interpretation
for
> > > > k=0. Why is it that one does not get the ideal case to come from
this
> > > > more general case when k=0?
> > > >
> > > > I hope that the question makes sense.
> >
> >
> > Not to me. The assumption is unphysical, in my view.
> > Cd is proportional to V^2 ??
> > What am I missing, would you say?

Brian Whatcott
Altus OK Eureka!