Re: [Phys-l] definition of gravity

[Bill Nettles <bnettles@uu.edu>]

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
> Sent: Tuesday, November 08, 2011 1:08 PM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] definition of gravity
>
> On 11/08/2011 11:39 AM, Bill Nettles wrote:
> > Are you saying that if one experiences an acceleration due to some
> > unknown interaction we must call it "gravity?"
>
> It is not the acceleration of "one" that matters.

[Bill Nettles] But "one" does accelerate doesn't one? Below you say objects (my "one") do accelerate. And I don't really understand what you mean by "matters." 

> It is the acceleration of the /frame of reference/.
[Bill Nettles] Okay, so correct the language and then answer the question. Can/should the acceleration of the frame of reference that is at rest with respect to "one" be due to a gravitational field?

> > What if, in reality, the acceleration is due to and EM interaction
>
> That's the wrong question.  The frame, being an abstraction, is not
> affected by EM interactions.

[Bill Nettles] 
[Bill Nettles] But the frame which is at rest WRT the object subject to the EM interaction is accelerated. Is it a gravitational field acceleration?
> ============
>
> The same issue arises in connection with centrifugity.  It exists in
> the rotating frame and not otherwise.
>
> If we have an object (such as a tetherball) going around and around in
> a circle, we all know that /the object/ experiences an acceleration.
>
> As always, we don't get to choose the data, but we do get to choose the
> model that we use to explain the data.  The choices include:
>
>  -- In the lab frame, there is no centrifugal field and the
>   coordinates of the ball are changing.
>  -- In a frame rotating at just the right rate, there is a
>   centrifugal field, and the coordinates are /not/ changing.
>  -- In a frame rotating at some other rate, there is some of
>   both.
>
> The amount of the centrifugal field depends on the rotation rate of the
> *frame* ... not of the object.
>
> Ditto for the gravitational field.  It is not affected by the motion of
> the object (such as the space shuttle).  OTOH it is strongly affected
> by the choice of which frame we use to analyze the motion of the
> shuttle.
>
> In my experience, students claim to understand and believe this the
> first time they are told, but they proceed to get it wrong again and
> again, and don't really understand it until about the tenth time.  This
> probably means that I am not explaining it properly.  I'm open to
> suggestions.  For now all I've got is this:
>
> The gravitational field depends on the acceleration of the /frame/ (not
> any particular object).
[Bill Nettles] This seems to make gravitational forces dependent on knowledge, i.e., if I don't know whether I'm having an EM interaction with the floor of an accelerating rocket or a mass-mass interaction with a planet or a charge-EM interaction, I'm going to call it "gravity?"
In SR (and GR?) we make a big deal out of the mass being invariant. Why can't we _define_ gravity as being Gm/r^2 and let accelerating reference frames modify this? Why call the contribution of the frame's acceleration gravity, too? 

> The centrifugal field depends on the rotation of the /frame/ (not any
> particular object).
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