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



-----Original Message-----
From: RAUBER, JOEL [mailto:JOEL_RAUBER@SDSTATE.EDU]
Sent: Friday, October 19, 2001 3:00 PM

[extracted from end of post]
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?)

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]

Part I: Spring-Scale operated in an equilibrium manner

What does it mean to be in an equilibrium manner? Equilibrium with what?
Do you mean dF/dt = 0? If so, that only makes the reading more difficult
to read without a snapshot. What am I missing?

[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.

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).

With each round, I get more confused over what is being debated.

--------------------------------------------
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
--------------------------------------------