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



-----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:


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?

I think that's right.

John Mallinckrodt

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

I've been a little endian for a long time now. You have succeeded in making me uncomfortable with breaking my eggs at the small end but I have yet to become comfortable with breaking them at the big end. I'm not trying to win any arguments here, I just want to find out a little bit more about this breaking eggs at the big end business. Thanks for your help. My discomfort with my old position, stems partly from the NIST definition--I like to go along with convention. In light of what you big endians have said however, it is some of my students' insistence on calculating the gravitational force in a statics problem involving an object at rest near the surface of the earth by applying F=ma with a being 9.8 m/s^2 that has contributed the most to my discomfort with the little endian model. The students don't draw a separate free body diagram but perhaps they are considering a hypothetical problem in which the object is in freefall and finding the force that would be needed to make the object's accelerometer reading be 9.8 m/s^2 and identifying that as the gravitational force on the object. (I don't think that they are treating the object at rest relative to the surface of the earth as having an acceleration of 9.8 m/s^2 directed approximately outward from the center of the earth but I have no excuse for not having asked.) I have been pushing the (little endian) idea that the g is an effective force per mass gravitational field vector (GM/r^2 with corrections--but I have not been emphasizing the corrections) and that the m is the gravitational charge such that the gravitational force on the object is W=mg, the product of the gravitational charge of the object and the force per gravitational charge. I am concerned that the students have a predilection to break their eggs on the big end and that it might not only make them more well-prepared to study general relativity if I teach the big endian model, but it might make it easier for them to understand how to solve statics problems. The right way of going about this is for me to read a book on general relativity, a subject I have never studied beyond the most superficial level. I plan on doing that, but in the meantime I would like to continue with the interactive learning experience made possible by phys-l. Based, however, on the use of the word "apparent" in N.M.J. Woodhouse's General Relativity lecture notes
http://www.maths.ox.ac.uk/~nwoodh/gr/
for the Michaelmas Term 2003:

Woodhouse> A celebrated 19th experiment by E¨otv¨os checked very
Woodhouse> accurately (to one part in 10^9) the equality of the
Woodhouse> two ms in the gravitational force on a mass and the
Woodhouse> centrifugal force due to the Earth's rotation: these
Woodhouse> are the two components in the apparent gravitational
Woodhouse> field.

it would seem that a good understanding of general relativity does not exclude one from the little endian camp.

Jeff Schnick