Re: non-inetial: example

["A. R. Marlow" <marlow@beta.loyno.edu>]

On Tue, 30 Apr 1996, Rauber, Joel Phys wrote:

> ...
> In full agreement with (2) above, an example of
> my statement that Marlow and I will get the answers.
> This kind of calculation is all I've ever claimed.
> ...

You can calculate kinematical quantities such as velocity fine  --  that
is not where your method goes bad  --  it goes bad when you use it to
calculate dynamical quantities: real forces acting, real work expended,
real energy use, real fuel consumption, accurate mass values, Etc.  Then,
if you allow introduction of pseudo forces, you must allow pseudowork,
pseudoenergy, and if your version of the second law is going to be 
talking about the same notion of force the third law talks about, you
must introduce pseudoobjects for the mysterious pseudoforces the third
law requires.

I repeat, any correct statement of Newton's laws (correctly relating
dynamical quantities with kinematical quantities) MUST include a 
reference to inertial reference frames, or the laws are simply not true
for the dynamical quantities.  I agree that this is all too often not
discussed clearly in many textbook treatments until the author feels
the need, and then it is often too late, because the damage has already
been done, but any reputable mechanics text eventually gets around to it,
and they all (whether explicitly or implicitly) include the proviso 
concerning inertial reference frames in presenting Newton's laws.

I also agree that, for purposes of relating purely kinetic quantities 
(position, velocity, acceleration) to each other, the matter of whether
you are going to introduce fictitious forces or not may be largely 
irrelevant, because your whole aim is to have purely dynamic quantities
cancel out anyway (whether real or fictitious),  but this is not at 
all the case when your aim is to calculate accurately the relevant 
dynamical quantities. Then introducing pseudoforces, pseudowork, pseudo-
energy is simply disastrous.

It would be akin to giving an astronomy lecture beginning "There are two
types of comets  --  real comets and pseudocomets. Some would call this
second kind "fictitious" but I am uncomfortable with that term, because
it tends to imply that they are not real, and they are really very real. 
(Some call the second kind  "meteors," but I don't agree with that because so
many people seem to want to call them comets. Think of the recent TV ads.)
So we can call them pseudocomets.  But just remember to be careful to
distinguish the two kinds, and, by the way, the second kind doesn't have
a tail, doesn't last very long and is in Earth's atmosphere, but other
than that they behave pretty much like comets. ..."

A. R. Marlow                          E-MAIL:  marlow@beta.loyno.edu
Department of Physics                 PHONE:   (504) 865 3647  (Office)  
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