Hi Russell,

Thanks for helping to settle the dispute.

I think it sounded like I was saying something I wasn't. I meant that
accelerometers do not measure gravity directly. You can certainly use
one to measure gravity, but only when certain other assumptions are
met. The only way it has of measuring acceleration is by compression
of some kind of spring, so the force must be transmitted THRU that
spring (which gravity itself is not but the force which acts against
gravity to hold something still IS).

For example, if you managed to charge both the proof mass and the body
of an accelerometer electrically, and then attracted it to an
oppositely charged object, it would not read the correct acceleration
either (since it is in a "force field" rather than under the influence
of an externally applied force). In general relativity there is
probably a neat, elegant, and profound (but not simple to understand)
way to state this but I do not know it.

Sean


On Wed, Mar 5, 2008 at 7:13 PM, Apptech <apptech@paradise.net.nz> wrote:
> >> If you are out in space and being attracted by the
>  >> gravity of a planet
>  >> nearby, accelerating at 1 m/s^2, the accelerometer would
>  >> read the same
>  >> thing as it would if the planet were not there.
>
>  > I disagree.
>
>  What you say is essentially correct, but you are disagreeing
>  with something that he didn't say or mean to imply.
>
>
>  > If the planet provided an acceleration of 1 m/s^2 due to
>  > it's gravity, and the accelerometer was actually
>  > accelerating at the
>  > same rate toward the planet,
>
>  Necessary condition as, if it were not accelerating at this
>  rate then some other forces would be involved
>
>
>  > then it's output would be zero (1-1=0).
>
>  Yes.
>  But
>
>
>  > If the accelerometer was subject to the same acceleration
>  > without the
>  > influence of the planets gravity (by e.g. a rocket motor),
>  > or was
>  > subject to the planets gravity but prevented from
>  > physically moving then
>  > it would most certainly be producing an output.
>
>  Yes, but that is not what he said.
>
>  First let's deal with what he said and then with why the
>  cases are different.
>
>  He said:
>
>  1.    Hang in "empty space". No masses within sensitivity of
>  detection of g meter. No meter reading.
>
>  2.    Hang in "empty space". Magically add planet close
>  enough to apply a 1m/s/s acceleration field. No meter
>  reading.
>
>  The above reflect what you would see.
>
>  ______________________
>
>  Now:
>
>  1.    Hang in "empty space". No masses within sensitivity of
>  detection of g meter. No meter reading.
>
>  2.    Start rocket motor so ship accelerates at 1m/s/s.
>  Meter reads 1m/s/s.
>
>  In the latter case the rocket motor reacts against the ships
>  structure and the g meter reacts against the ship. The
>  forces are not applied to the whole structure. If you could
>  find a way of applying the forces that the acceleration
>  "field" applied to all objects within the ship
>  simultaneously then a g meter will not see the acceleration
>  relative to itself.
>
>
>
>         Russell McMahon
>
>
>
>
>
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