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



Thanks Robert for paying attention.
And making my point succinct and clear. (Whether you agree or not.)


On Saturday, October 20, 2001 5:24 AM, John S. Denker wrote:

At 05:09 PM 10/19/01 -0500, RAUBER, JOEL wrote:
one cannot use the spring to measure other forces, ***without other
kinematical measurements***

1) I am mystified by the reference to "other" forces.
What other forces? Other than what?

The spring-scale is advertised to measure the force applied to its
hook. Nothing more, nothing less. (There will of course be
imperfections
and nonidealities, but that doesn't change the concept.)

John,

Let's examine the object that is applying the force to the
spring's hook.
There may be other forces acting on the object besides that applied by
the spring.


Which is what I thought was the point of using the spring scale to begin
with, to investigate those other forces.

1. Is it possible to determine the magnitude of the other forces
on the object only knowing the force as measured by the spring?

Apparently, this is not the question you are interested in.
However, I
think
you will agree that it is not possible without using F=ma
and, consequently,
other kinematical measurements (to get a).


Precisely my point.

A separate question is "what is the point of question #1?"

I think the answer has to do with determining forces such as
Fg, Fe, etc.,
but perhaps Joel can help us out here.


Exactly. And you are right, it is a seperate question; but the point hinges
on agreement regarding what one is doing when you use a spring-scale and
F=ma; hence the degeneration of John D's and mine "discussion".

Have you had a chance to read my "RESET" post?

I'm not sure if that post helps explain the point of question 1 or not. If
not I'll try to expand on the issue.

The one sentence version is that it is a question related to interpretation
of F=ma;
namely, can F=ma be interpreted as an operational definition of force from
measurement of acceleration (I think agreed to by all); and in a stronger
form; as an operational definition, must it be interpreted this way?

The only clear alternative I have seen yet on the discussion has been:

a) Equilibrium measurements as an independent means for obtaining (measuring
forces). This is why spring-scales were brought into the conversation, or
so I thought.

b) John D. mentioned what I interpreted as another possibility (but I'm not
sure, as the discussion degenerated in an unfortunate manner at that point,
mea culpa); namely operating the spring-scale in a non-equilibrium manner,
in order to measure other forces.

(parenthetically, I don't really know if that was the intended point of what
John D. said, as I inferred from subsequent posts, that the example was
meant to illustrate that whether or not the spring scale pointer is moving
(or accelerating), the force of the spring on the point is never-the-less
k*X. Something I don't dispute. Regardless of intent, the non-equilibrium
use of the instrument still has bearing on what for me is the interesting
question.)

I don't think it helps to talk about both (a) and (b) at the same time. So
I wish to restrict ourselves to (a) at first and tackle (b) later. Since,
if there is no agreement on (a), why bother discussing (b)?