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Hi all,
I've been following this thread for a bit but haven't really taken the
time to think about the situation until now, and here's what I came up with:

I would argue that the best way to describe what the accelerometer
measures is that it measures the difference in net forces between the
ball and the outside casing (assuming the basic model of a ball
connected to a spring connected to the casing).

When you're in open space with nothing around, net force on both is zero.

When a rocket engine pushes the accelerometer in open space at 1g, the
rocket body applies force to the casing but not the ball. Hence, the
accelerometer measures the 1g difference.

When the accelerometer is in freefall towards a planet pulling at 1g,
the 1g gravitational force is applied to both the casing and the ball,
and the accelerometer measures a difference of zero.

When the accelerometer is sitting on a table in a g-field of 1g, the 1g
gravitational force is applied to both the casing and the ball, but the
table's normal force of 1g is applied only to the casing and not the
ball. The accelerometer measures the difference of 1g.

I hope this is sufficiently elegant! It definitely does eliminate the
apparent distinction between gravity and "everything else" - the
distinction is that gravity pulls on the ball and the casing, while
other forces push or pull only on the casing.

Chris

Sean Breheny wrote:
| 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
|
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