>> 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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