Re: Feeling Acceleration

["A. R. Marlow" <marlow@beta.loyno.edu>]

> I don't have too much trouble with the above; I think most respondents
> here have meant to say something like, "We sense internal forces or
> pressures that are generated in response to the forces that cause us to
> accelerate."  This is why I prefer to say simply, "We experience
> acceleration."
>
> On the other hand, acceleration is not quite as arbitrary as you imply. 
> The principle of equivalence teaches us that there are preferred reference
> frames--those in which there are no local gravitational fields.  We
> experience acceleration whenever we are not in one of those reference
> frames. 

I find the use of "experience" here a little ambiguous:  if you mean it
simply as a synonym for "We undergo acceleration" then of course there is
no problem.  On the other hand, if you intend it as a synonym for "sense"
(as with inner ear or skin sensors), then I have the same difficulty as 
before.  We simply do not sense the kinematical quantity acceleration 
(defined in terms of meter sticks and clocks relative to a specific
frame);  we sense the dynamical quantity pressure, or equivalently, force.  


> ....  For instance, the sensations we experience when
> accelerating upward at 2 m/s^2 with respect to the earth's surface are far
> from double what we experience when we accelerate at 1 m/s^2.  The ratio
> is more like (9.8+2)/(9.8+1) = 1.093.

My point precisely!  Our sensations register exactly the force we
experience, and not at all the acceleration.

> ... acceleration with respect to local inertial frames.  Now the
> correspondence between acceleration with respect to inertial frames and
> our sensations is simple; when our acceleration is twice as great we
> "experience" twice the sensation.

All very true, and the "sensation" experienced is exactly the force
involved, and not the kinematical quantity acceleration. 

> ...
> Accelerometers are nonsensical devices indeed within the framework of
> Newtonian gravitation.  But they are simple to understand and very useful
> within the framework of a theory of gravitation based upon the principle
> of equivalence like GR. 

I made no claim that accelerometers are "nonsensical devices" and I
certainly would not claim that they are not useful.  It's just that,
unless they are purely kinematical instruments for measuring acceleration,
they are confusingly named, because what they are registering is force.


A. R. Marlow                          E-MAIL:  marlow@beta.loyno.edu
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