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Re: The drag force -- a correction cubed



At 13:26 10/7/00 -0700, John Mallinckrodt wrote:
/// Jack was right; there is no need to add Ludwik's
restrictions. It *is* a fact that, at their various terminal
velocities, the drag forces on *any* collection of objects will be
proportional to the masses of those objects (since they are
*equal* to the gravitational forces on those objects.) This does
*not*, however, imply that drag forces are, in general,
"proportional to" the mass of the object to which they are
applied. I'll go out on a limb (a short and sturdy one, I think)
and note that they don't even *depend* on the mass. Drag forces
depend on the velocity, shape, size, surface characteristics, and
attitude of the object and also on properties of the medium
through which the object moves. They never depend fundamentally
on the object's mass.

I felt John did an admirable job in comprising two opposite views
in his synthesis until this point.
Thereafter however he errs, in using a term with a technical
definition and meaning, but in a looser 'physics' rendering
as follows, and indicated by '>...<':



Notwithstanding all of the above, there is nothing "wrong" with
*expressing* a >drag coefficient<
as the product of the object's
mass with another positive constant.
///
John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm


This term, drag coefficient, in its technical (aerodynamics) sense,
is not to be understood as a product of mass and a constant:
it is a shorthand for size, texture and shape as they impact airflow.

I expect the phrase John had in mind is 'drag force', misleading
though it is in the context of gravity force balancing.



brian whatcott <inet@intellisys.net> Altus OK
Eureka!