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Re: [Phys-l] definition of gravity



On 11/08/2011 08:12 AM, Robert Cohen wrote:
Follow-up question: does it matter whether I say "Earth's
gravitational field" or "locally measured gravitational field"? For
example, is it appropriate to say the Earth's gravitational field is
zero at the location of the space shuttle orbit?

I wouldn't recommend it. The field the astronauts observe
locally, in the frame comoving with the space shuttle,
would more aptly be called the shuttle's gravity than the
earth's gravity.

On 11/08/2011 08:46 AM, LaMontagne, Bob wrote:
What name do you give to that part of the" gravitational field"
solely due to the presence of a nearby gravitating object (such as
the Earth)?

That is a seriously good question. I've struggled with that.
*) In many applications it makes sense to call it the /primary/
gravity (g_I).
*) Sometimes I call it the /Newtonian/ contribution to the
gravity, in honor of Newton's law of universal gravitation,
and of his application of that law _in a nonrotating frame_
to the orbital dynamics of the moon.

If anybody can come up with a cleverer name, that would be
great!

On 11/08/2011 08:54 AM, John Mallinckrodt wrote:
I don't know what that "part" is because there is no way of measuring it.

There *are* ways of measuring it.
-- You can't measure it using local, first-order measurements.
++ You can easily measure it using either non-local measurements
or higher-order measurements.

Specifically:
-- The gravitational acceleration (g) leads to an energy that
is first order in position. This is indistinguishable from
a uniform acceleration of the reference frame.
++ The tidal stress leads to an energy that is second order in
position. This is unaffected by a uniform acceleration of the
reference frame.

This is very similar (but not quite identical) to the fact that
locally to first order you cannot distinguish a centrifugal
field from a Newtonian gravitational field ... but non-locally
and/or to second order you can.