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Re: [Phys-l] g...



On 11/19/2006 06:33 PM, Folkerts, Timothy J wrote:

To me, the strange thing about the NIST definition is the terminology "local force of gravity".
Apparently "force of gravity" is NOT well-established!

What's strange about that? The physics requires it. Force is
a vector. The vector direction is clearly different "here" versus
the antipodes. The vector magnitude is a function of altitude.

From NIST:
"The local force of gravity on a body, that is, its weight, consists of the resultant of all the
gravitational forces acting on the body and the local centrifugal force due to the rotation of
the celestial object."

That makes sense to me. That's the definition I've been using
for a long time.

The "local force of gravity" is not the gravitational force at a particular location??? So NIST
is defining a whole new term - the local force of gravity - which is the same a weight (which is
the same as many people call "apparent weight). To define "the local foce of gravity" as
including gravity AND centripetal forces just seem perverse to me.

It seems non-perverse from the perspective of the equivalence principle.

It also agrees with conventional practice. When weighing things, I have
/never/ seen anybody correct for centrifugal acceleration, even in cases
where it would have been quite significant.

It also agrees with the convention of measuring altitude relative to
sea level. The sea is well approximated as an isopotential if and
only if you include the centrifugal field as well as the GmM/r^2
gravitational interaction.

Also the NIST terminology is conventional. Local does not refer to
a particular locality, but rather to a particular term in the Taylor
expansion. My perspective on this can be found at
http://www.av8n.com/physics/rotating-frame.htm#sec-grav-elec