Re: Feeling Acceleration

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

brian whatcott says:
> At 01:49 AM 4/10/97, James Mclean wrote:
> ...
> > The accelerometer in an auto air bag measures the auto's acceleration
> >   without measuring the force on the auto.  If the auto's mass were known,
> >   then the device could subsequently figure the force.
> > A spring scale will measure the force between two bodies, but doesn't give
> >   any indication as to whether either body is accelerating.  But given
> >   additional information, an acceleration could be deduced.
> >
> > Some measurements are more indirect than others.
>
> I feel a little uncomfortable about this airbag sensor text.
> I seem to recall there is a  mechanical switch type that uses a ball restrained
> by a spring, which allows the ball to move onto a contact, given sufficient 
> deceleration.

I think that's basically right.  It gets to Donald's point that what is
being measured depends on how deeply you want to look into it.  At the
level you describe, the switch is measuring a force between a small weight
on a spring and some superstructure.

At the level of treating the sensor as a black box, however, it is correct
to say that the sensor measures the acceleration of the car but does not
measure the force on the car.

One could imagine a larger black box which included circuitry which did a
calculation using the known mass of the car to read out the force on the
car.

Some measurements are more indirect than others.
A question which follows: is there a "least indirect" level that we can
consider, or are we destined to always be peeling back one more layer?

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