Re: Force sensors (2)

["Rauber, Joel Phys" <RAUBERJ@mg.sdstate.edu>]

> 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.

agreed concerning velocity of the two ends of the spring (or equivalently 
the pointer relative to gradations); I misunderstood the comment before.  I, 
of course, meant that the overall velocity of the spring and apparatus could 
have non-zero velocity relative to some quasar.   I still have to think 
about the "jerk" though; it strikes me that you are adding an additional 
requirement to the usual conditions of 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.

Measuring the elongation over time to be constant, does tell me the 
acceleration over that is zero, and hence can be viewed as a measurement of 
the acceleration.  The elongation is a determination of position and if that 
is constant in a coordinate system, the acceleration is zero for that 
position in the the said coordinate system.


> Principally,
> the observation of "no change" does not require the ability to perform the
> subtraction required for taking derivatives.