Re: [Phys-l] definition of gravity

[John Denker <jsd@av8n.com>]

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.
It is the acceleration of the /frame of reference/.

> 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.

============

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).

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