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# Re: [Phys-L] two very different "gravity" concepts

• From: John Denker <jsd@av8n.com>
• Date: Wed, 02 Jan 2013 22:06:24 -0700

On 01/02/2013 05:33 PM, Bob Sciamanda wrote:
The intended implication is not that the frame is "freely falling" but that,
in the observer's frame, the observed mass m is free to "fall".

Well, even then, that doesn't answer the question that I
meant to ask. IMHO we need a word that describes g, the
g that appears in the equation F = m g. This g depends
on what the *frame* is doing, not on what this-or-that
mass is doing.

To illustrate this point, consider two masses and two
frames, making four cases altogether:

falling mass, falling mass,
elevator frame lab frame
g = 0 g = 9.8 m/s/s

mass on lab shelf, mass on lab shelf,
elevator frame lab frame
g = 0 g = 9.8 m/s/s

Note that the elevator is in free fall, and that the
shelf is fixed in the lab frame.

In all cases, g depends on what the frame is doing,
not on what this-or-that mass is doing.

This is the attraction to calling g the framative
acceleration of gravity, because it is frame-relative.

The framative g is distinct from G M / r^2, which is
not a frame-relative quantity.