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Re: [Phys-l] Weightless (running around in circles)



John M., Do you buy the following?

It seems to me that the big differences between the GR model and the
Newtonian model have to do with what an inertial reference frame is and
what acceleration is.

A Newtonian inertial reference frame is something that can only be
imagined. One way of doing so would be to imagine a non-rotating
spherically symmetric object with an ideal accelerometer at its center.
Have it be and remain at a location in space at which the total GM/r^2
gravitational field from all matter in the universe is and remains zero.
(To me, establishing that this is the case is the impossible part.)
While the object is at such a location, as long as the object's
accelerometer reading is zero, any reference frame in which the object
is not rotating and the object's r double dot is zero is an inertial
reference frame. As viewed from any such reference frame, the
acceleration of any object is its r double dot, its Newtonian
acceleration. The Newtonian acceleration of an object is always
accompanied by a net force that is directly proportional to the product
of the mass of the object and the Newtonian acceleration of the object.
The force could be a GMm/r^2 force. A net force is occurring when the
object is interacting with its surroundings.

An Einsteinian inertial reference frame seems to be much simpler. Our
non-rotating spherically symmetric object represents an inertial
reference frame whenever its accelerometer reading is zero. The GR
acceleration of such an object would be its accelerometer reading. A
net force on this object would be an ongoing interaction with its
surroundings that is accompanied by a non-zero accelerometer reading.
There is no GMm/r^2 force because the mere existence of other massive
objects has no effect on the already-zero accelerometer reading.

In the Newtonian model, the r double dot of an inertial reference frame
relative to another inertial reference frame is always zero. Not so in
GR. In fact, GR is a way predicting what that r double dot would be.
In GR with Newtonian language, one can describe the r double dot of an
object whose accelerometer reading is zero, relative to another object
whose accelerometer reading is zero, in terms of a pseudo force, and we
can call that pseudo force a gravitational force, but it is not a GR
force because it doesn't change any accelerometer readings.

Jeff Schnick

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Mallinckrodt
Sent: Friday, November 24, 2006 6:07 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Weightless (running around in circles)

On Nov 24, 2006, at 2:31 PM, Jeffrey Schnick wrote:

Suppose I established a coordinate system and took some position
vs. time measurements on the object that has the accelerometer
attached to it. Further suppose that, from that data, I determined
the acceleration of the object, call it r double dot, relative to
the established coordinate system.

O.K. so far.

I understand r double dot to be the what NIST calls "the local
acceleration of free fall in that reference frame."

? I sure don't follow that. I would agree with you if the object
actually WAS in free fall, but I didn't see that specified. Maybe
you simply forgot to say so?

Is the NIST definition of weight just how hard and which way I
would have to
push on the object to make its accelerometer reading be r double
dot?

Assuming the object was in free fall when you measured r double dot,
I think that's right.

John Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://outlawsofphysics.com>



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