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



Okay, I think I see JR's point. You are not saying that one cannot measure
force with a spring but rather one cannot use the spring to measure other
forces, right?

So, for example, if I want to identify the force of gravity, I would need to
have a=0 so that I could use F=ma to get Fnet=0 and thus the other force?

If this is a correct interpretation, then I think JR and JD are talking
about two different things. JR's argument would be essentially that if we
only know one of the forces acting on the object, we cannot infer the other
forces acting on an object without invoking F=ma (and thus a measurement of
a). JD's argument, on the other hand, seems to be that we can use the
spring scale to calibrate other spring scales regardless of the acceleration
measurement (since F12=-F21).

Is this right? Or have I completely missed the point? (which wouldn't
surprise me)

--------------------------------------------
Robert Cohen rcohen@po-box.esu.edu
570-422-3428 http://www.esu.edu/~bbq
Department of Physics
East Stroudsburg University
East Stroudsburg, PA 18301
--------------------------------------------

-----Original Message-----
From: RAUBER, JOEL [mailto:JOEL_RAUBER@SDSTATE.EDU]
Sent: Friday, October 19, 2001 4:29 PM
To: PHYS-L@lists.nau.edu
Subject: Re: operational F, m, and a (velocity measurements with
fish-scal es)


Quick reply to Robert C:

Thanks for "scaling-in"

-----Original Message-----
From: Robert Cohen [mailto:Robert.Cohen@PO-BOX.ESU.EDU]

I think I agree with JD - I still don't see why we need to know the
velocity of anything to get k*x (if k is already known).

[extracted from beginning of post]


I think we need to clear-up the meaning of "use of the scale in an
equilibrium manner" before addressing your statement of support for
obtaining k*x.

Part I: Spring-Scale operated in an equilibrium manner

What does it mean to be in an equilibrium manner?

How do you use a spring scale? In particular, how do you use
it measure
some unknown force? (I'm thinking of one those spring scales
that often are
calibrated to measure in Newtons and are almost ubiqitous
either in intro
labs or their store-rooms).

The way I use them is to place the spring force, provided by
the spring
scale, and place it in opposition to the unknown force I am trying to
measure; and in such a manner that the pointer of the scale
is in mechanical
equilibrium (meaning it isn't moving relative to the tick marks of the
scale).

Isn't this what almost everybody does? What am I missing?

The above procedure is what I mean (I hope) by using the scale in an
equilibrium manner.

Equilibrium with what?

Mechanical equilibrium between the spring force and the
unknown force you
are trying to measure.

Do you mean dF/dt = 0? If so, that only makes the reading
more difficult
to read without a snapshot. What am I missing?


I don't think so.

[snip]

Part II: The use of a spring scale in a non-equilibrium manner
[snip]
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!

Hmmm...up until now I thought you two were using the spring to get
the force exerted *by* the spring on an object and then comparing
that to the mass and acceleration of the *object* (although for
a fish scale we need to recognize that there are other forces, like
gravity, also acting on the object). Of course, it makes no
difference to use the spring to get the force exerted *on* the
spring and compare that to the mass and acceleration *of* the spring
but then setting the mass of the spring to zero seems a bit
confusing
since we usually attach the spring to something very massive.


This may be a source of confusion between John D and me. I
agree that if
the spring scale is in a state of uniform motion relative to
an inertial
reference frame, that looking at a snapshot picture and
obtaining x, is
sufficient for determining the value of the spring force on
the object its
attached to at that instant of time. But that seems to me as
tantamount to
saying I can use the spring scale to determine the force that
the spring
scale exerts on an object. I have no arguement with that. (I'm still
worried about non-inertial reference frames, but lets let
that lie for a
moment.)

But that's not why I thought spring scales were brought into the
conversation in the first place. I thought they were brought into the
discussion for purposes of using them operationally to determine other
unknown forces without acceleration measurements.

(more in another post to come).


Either way, it seems to me we should be able to get k*x
from a picture
of the scale, regardless of whether the spring and/or object is
accelerating. For an object being weighed, however, we need
to recognize
that the net force on the object may not be zero (in which
case, it will
oscillate).

Which means what if I'm doing this to measure the
gravitational force on
that object?
Meaning can I do it with the oscillating pointer with just a single
snapshot?

I don't think plugging in the instantaneous x value of the pointer and
saying mg = k*x and then infering that that is the magnitude of the
gravitational force is going to do it!