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

[John Denker <jsd@av8n.com>]

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.