Re: [Phys-l] Equations (causal relationship)

["David Bowman" <David_Bowman@georgetowncollege.edu>]

Regarding George Spagna's observation:
 
> Brian McInnes wrote:
> ...
> > One (or more) force(s) acts (act) on a particle; a particle 
> > experiences a single acceleration (which may have a different value 
> > in a different frame of reference).
>
> Without entering the ongoing debate, I note that the acceleration
> will be the same for all observers in inertial reference frames as
> long as we're in the regime of classical (non-relativistic)
> mechanics.
 
Although I, too, don't wish to become embroiled in the debate
concerning the relationship of Newton's 2nd law to 'causes' &
'effects', I nevertheless wish to point out that although George's
observation above is true, I don't think it is typically very
helpful.  This is because often some of the reference frames of most
interest in a problem are often not inertial.  In the case of the
controlled accelerating car example that John M. and others have been
discussing the two most relevant reference frames are the one in
which the local surface of the Earth is at rest and the one in which
the vehicle and its occupants are at rest.  In the case of the
aviation problems for which John D has a particular fondness there is
also a relevant reference frame in which the local ambient air is at
rest.  (This local fluid-medium-at-rest frame is also relevant for
any other problems involving such things as drag resistance, lift,
the acoustic Doppler effect, etc.)  None of these reference frames
are always particularly inertial (especially once the implications of
the Equivalence Principle are considered).  Once we start performing
transformations to or from various noninertial reference frames we
don't have any reason to consider acceleration as an invariant of the
transformation.
 
David Bowman