Re: operational F, m, and a (velocity measurements with fish-scal es)

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

Maybe some minor progress, see below.  (maybe not)

Part I: Spring-Scale operated in an equilibrium manner
__________________________________________________
> At 11:32 AM 10/19/01 -0500, RAUBER, JOEL wrote:
> > If you are operating it in an equilibrium manner, have you not made a
> > velocity measurement of the pointer relative to the tick-mark scale?
>
> Whether or not I have made a velocity measurement is irrelevant.  The
> position matters;  the velocity does not.  To keep me honest,
> let my lab
> technician make the measurement.  He then emails me the x
> reading but not
> the (dx/dt) reading.  I still know the force.

When the technician operates the scale in an equilibrium manner:

Are you telling me that your technician is not staring at the pointer for a
time interval Delta t, in order to make sure that the pointer isn't moving,
so that he knows what number to write down, before he writes down the value
of x that he e-mails you?

If not, how does your technician know whether or not the spring scale is in
equilibrium?  A single snap-shot picture doesn't tell you.  (Remember: for
the moment we are operating the device in an equilibrium manner.)

> This is a PHYSICS experiment that tells me that the pointer-position
> matters and the velocity does not (to an excellent
> approximation for any
> halfway-decent spring-scale).

see comment above

> Any observation of the velocity is an irrelevant accident, a
> red herring, a
> distraction.

whether it is relevant or irrelevant is under debate.  Actually for me the
question is whether or not the velocity measurement is a necessary part of
using the apparatus.  If its necessary its relevant. If its not, its
irrelevant.

> > The experimental evidence is the evidence of how everyone I have ever
> > chatted with uses a spring scale.
>
> This is not physics.  This is some form of sociology, and not very
> systematic sociology.

Isn't observation a necessary part of experimental science?

Furthermore, it is how most people learn things, you observe how the many
people around you are using the apparatus, doing the procedure, understand
the topic; and then you test it for youself, (trying it out for yourself).
Its how I learned to talk, to walk, to use spring scales, to use rulers, to
drive a car, my understanding of Lagrangian mechanics etc.

________________________________________

Part II:  The use of a spring scale in a non-equilibrium manner

We now turn to operating the spring scale in a non-equilibrium manner.

> Everyone _I_ have chatted with recognizes the parallels
> between the spring
> scale and the mass-on-a-spring oscillator.  They have no
> trouble writing
> down the equation of motion that involves a force dependent
> on the POSITION
> (i.e extension) of the spring, independent of the velocity,
> even when the
> mass is oscillating, i.e. far from equilibrium.

I have had the same chats and have arrived at the same conclusions.  Now,
how do you decide what number to put down for x in your data book?  And how
are you going to convert that to a value for spring force?  Well, you have
described how in a previous post.  I quote:

"1) Fundamental point:  Even though there may be other contributions, there
is still SOME force attributable to the IDEAL contribution (k x)."

"2) In practice, the nuisance contributions (gamma v) and (m a) can often be
negligible, even if (v) and (a) are distinctly nonzero.  A good
spring-scale will have good bearings and good lubrication, so that gamma is
small.  Similarly, assuming (a) is not too huge, it is easy to arrange that
(m a) is small compared to (k x), since the spring in a good spring-scale
doesn't have much mass (m)."

"3) Furthermore, if you have even a rough notion of (gamma) and (m), you can
make a high-velocity high-acceleration observation and then, during the
subsequent data-analysis phase, correct for these nuisances.  There will
remain SOME force attributable to the IDEAL (k x) contribution."

In item (3) you state that it is necessary for you to correct for these
nuisances, in order to achieve arbitrary accuracy.  Since we are discussing
the use of the instrument in principle, I assume that such requirements of
arbitrary accuracy are germane.

If not, then we have no disagreement; but then we are no longer talking
about whether or not a velocity or acceleration measurement is implicit in
the use of the instrument as an ideal device.

> Physics does not restrict springs to producing forces only in
> equilibrium.  Any such restriction exists in the imagination
> of those who
> wish to imagine it.

I imagine that it takes a great imagination to imagine that I have, or so I
imagine.  :-)

I never placed any such restriction.  Which is why I have several times
agreed that you may, and in fact have, described a means of operating the
spring scale in a non-equilibrium manner.

I think it would be helpful to remember that we aren't using the spring
scale to calculate k*x, we are using the spring scale to measure some other
force that we are presumably putting in opposition to the spring.  This is
the whole purpose of using the spring scale to operationally define forces!

We may be in dire need of reset on this discussion.

Is anybody else brave enough to weigh in?  (or should I say scale in?)