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

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

Michael:

I don't think I have issue with anything you said in the below post.

humour mode = on

(except, perhaps, with the "then I think I started agreeing with John."
part)

:-)

humour mode = off

The fact that I have no issue with what you are saying below is why I think
John D and I are talking about different issues.  At least in part.

In the context of what you wrote, imagine that one has taken this
sufficiently ideal spring scale to the surface of the Moon, and make the
determination ". . . that the 0.10194 kg mass doesn't bring the pointer
anywhere close to the 1 newton mark".  I'm questioning what is necessarily
involved in making that determination; what does one do to actually decide
that, and lets even try to measure the force on the moon (not just one tick
mark, but calibrate the spring scale with a whole bunch of tick-marks;
presumably done on the earth, but that doesn't matter I think).  What is one
actually doing when you measure that force on the surface of the moon with
your scale?

In other words, its what you haven't said that I'm concerned about.

Joel R.

> -----Original Message-----
> From: Michael Edmiston [mailto:edmiston@BLUFFTON.EDU]
> Sent: Friday, October 19, 2001 4:41 PM
> To: PHYS-L@lists.nau.edu
> Subject: Re: operational F, m, and a (velocity measurements with
> fish-scal es)
>
>
> In some of my early readings of John D's messages, I think I
> was misreading
> him, and it may be others are still doing a thing similar to
> what I was
> doing.
>
> I was thinking we had to have the spring scale provide the
> same reading for
> a particular force as we would measure if we also used F=ma,
> or F=Gmm/r^2,
> or some other method for measuring that force.
>
> But then I realized that was not what John was trying to say.
>  If the spring
> scale would be the primary standard, and when it is stretched
> to its mark
> that would be the definition of one newton, then this would always be
> correct... if we can idealize the spring and pointer and hook to have
> insignificant mass.
>
> We could make it so that when we are in a nearly inertial
> reference frame at
> a location on earth where g = 9.81 that the scale reads 1
> newton when a
> 0.10194 kg mass is hung on it.  But if we do that, and then
> take the scale
> to the moon and find that the 0.10194 kg mass doesn't bring
> the pointer
> anywhere close to the 1 newton mark, the scale is not wrong.
> In fact, it is
> correct, we don't have a one newton force on it.  We're not
> supposed to be
> trying to measure mass with the scale.  We're trying to
> measure force.  The
> scale will do that properly whether it is in earth's g, or
> moon's g, or zero
> g, or accelerating, or moving with constant velocity, or
> whatever... as long
> as the spring's mass and pointer's mass are small enough to be of no
> concern.
>
> If the spring's mass, pointer's mass, hook's mass are indeed
> significant,
> then it seems to me we would have a problem in various g
> fields or various
> non-inertial frames because some of the spring deflection
> would be caused by
> the scale's internal components as opposed to the externally
> applied force.
>
> Maybe everyone has already agreed to this, and you are all discussing
> something different.  But this was a problem for me.  Once I
> idealized a
> massless frictionless mechanism then I think I started
> agreeing with John.
>
>
> Michael D. Edmiston, Ph.D.              Phone/voice-mail:
>   419-358-3270
> Professor of Chemistry & Physics        FAX:
>   419-358-3323
> Chairman, Science Department            E-Mail
>   edmiston@bluffton.edu
> Bluffton College
> 280 West College Avenue
> Bluffton, OH 45817