Re: operational F, m, and a

["RAUBER, JOEL" <JOEL_RAUBER@SDSTATE.EDU>]

> At 09:44 AM 10/17/01 -0500, RAUBER, JOEL wrote:
>
> > I still think there is an implicit acceleration measurement occuring,
> > however.
>
> No, not in any reasonable sense.

Please explain?

If you mean by "reasonable sense", what I actually do at the vegetable
counter of the grocery store, than OK.  But I'm looking at Gedanken
Operations related to the foundations of mechanics, where I think such
accelerations are very important.

> > .... in order to use the fish scale to determine force in the context
> > of Newton's second law I have to make sure that the tick
> mark scale on the
> > apparatus is a good inertial frame of reference, which means I must
> > implicitly make another kinematic measurement, namely that
> the tick mark
> > scale has zero acceleration relative to some fiducial
> inertial frame of
> > reference.
>
> No.  All you need is a snapshot showing the fish-scale
> pointer relative to
> the fish-scale graduations.  If the whole system is
> accelerating past you
> at the time of the snapshot, so be it.

I don't know what you mean by a snapshot, if you literaly mean a picture at
one instant of time than I don't know that the fish scale was in equilibrium
by looking at a "snapshot".

The acceleration of the fish scale (or me) is very important if we are
talking about forces found in the usual statement of Newton's second law.

> > If you buy this, it means that equilibrium measurement
> determinations of
> > force involve measurement of acceleration.
>
> Nope.  Why make it complicated?  For pedagogical purposes, especially
> introductory purposes, it suffices to consider static
> situations.

I'm not discussing this at the moment for introductory purposes; or for
pedagogical purposes for the introductory classroom.  Hence, what is found
below is irrelevant to the discussion (or at least the point I'm trying to
make, and socratically discuss with you and fellow members of the list;
which at this point is a thread discussing the fundamental foundations of
Newtonian Mechanics through Newton's laws.)

> Start
> simple:  hang a weight from a fish scale.  No acceleration.
> Or pull on one
> fish scale with another.  No acceleration.  If you want some useful
> complexity, illustrate the vector character of the force
> concept by hanging
> a mass from two fish scales, using strings at funny angles.
> No acceleration:
>
>          S
>           \     S
>            \   /
>             \ /
>              |
>              |
>              |
>              M
>
> Talking about the acceleration of the fish-scale pointer is a
> distraction
> at best.  The spring produces a force even when the pointer is not
> accelerating relative to the graduations, so F=ma is clearly
> irrelevant to
> the primary function of the fish-scale.  (Even in the
> perverse case when
> the pointer is accelerating relative to the graduations, this
> introduces
> only a small correction, assuming the mass of the fish-scale
> mechanism is
> small compared to the mass of the objects being weighed.)
>
> Saying that the fish-scale depends on F=ma makes about as
> much sense as
> saying that force equals mass times optical wavelength,
> because you can't
> see the pointer unless you know what color it is.  By that I mean that
> color and pointer-acceleration have only the most remote,
> tangential, and
> superficial relationship to the normal operation of the fish-scale.