Re: Force sensors (2)

[jmclean@chem.ucsd.edu (James Mclean)]

Rauber, Joel              Phys says:
> > but the converse doesn't
> > work: acceleration = 0 is not sufficient to conclude that equilibrium has
> > been reached.
>
> > The velocity, jerk, ... must also be zero.
>
> The velocity does not have to be zero for the system to be in equilibrium.
We are refering here to the velocity of the spring scale indicator relative
to the gradations it points to.  That velocity certainly has to be zero for
equilibrium.

> > > 1) You put the spring in equilibrium.  This is the step where you measure
> > > the acceleration.  You must measure the acceleration to be zero in order 
> > > to know that the spring is in equilibrium.
> >
> > Actually, you must measure the spring elongation to be constant over time.
> > This does mean that the acceleration is zero,
>
> And hence is measuring the acceleration, i.e. this is how you operationally 
> use the apparatus.  You look for the necessary condition that the 
> acceleration is zero.

Measuring the elongation to be constant over time is not the same as
measuring the indicator's accleration, for several reasons.  Principally,
the observation of "no change" does not require the ability to perform the
subtraction required for taking derivatives.  Consider the following
sequence:

ahfjs
ajgdj
ajfsl
ajfsk
ajfsk
ajfsk
ajfsk  ...

I can tell you that whatever it is, it has become constant.  But I have no
idea how to define a derivative for these symbols (Well, OK, I could come
up with one, but I don't need to.)

-- 
--James McLean
jmclean@chem.ucsd.edu
post doc
UC San Diego, Chemistry