Gene:
While your point is well taken, such considerations (frame of reference,
black hole, Einstein, ..) were deliberately left out of the chapter. The
preface to the book says that. (If such were included, the book would
probably
still be on the shelves unsold.)
The chapter in question deals with the selection of force and mass units
for the value of "weight," "load" and similar terms. With weightlessness,
and any degree of it, the unit is the newton, regardless of the frame of
reference or anything else. That's all that is to it. Your point is not
pertinent to the selection of the unit. If included in, it would likely
muddle the weight issue for the intended reader more than clarifying it.
Thanks for adding this detail for the few USMA audience who might be
interested in weightlessness itself. My book as well as SI 10 is supposed to
explain SI for the units and their usage, not to teach physics.
I should like to add that the CGPM involvement in the weight issue is silly,
and even SI 10 would be better of leaving that issue out (or stick it into
an
appendix). It is a "problem" in some cultures/ languages only, not an
international
standard matter. For example, in Czech, weight as mass is "hmotnost," weight
as gravity force is "vaha," weight as in counterweight is "zavazi" (or
protizavazi) - no conflict of meaning and units.
I would vote against the inclusion of the frame of reference and other such
unit-impertinent subjects in SI 10. The weight issue is an I-P (slugs!)
problem not SI.
Stan
----- Original Message -----
From: <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>; <[email protected]>
Sent: 07 Sep 01, Saturday 20:26
Subject: Mass and Weight in Jakuba Book
Stan,
In Classical Mechanics, position, speed, and acceleration *must* be with
respect to a particular "frame of reference"; that is, with respect to a
particular "coordinate system" if position, speed, acceleration, *and
weight* are to be measured without ambiguity. There must be an origin, and
definitions of three coordinates (rectangular, cylindrical, spherical, or
three coordinates from some other of more than ten sets) for locating
positions in space.
Classically (no speeds of physical objects approaching c, the speed of
light), mass is independent of position, speed, and acceleration. In this
case, one may write force = mass times acceleration, Newton's Second Law
in its most simple form.
However, weight (defined in SI as mass x acceleration of gravity)
*requires* either specification or unambiguous knowledge of the particular
frame of reference in question.
This necessary requirement for a "frame of reference" is *missing* in your
discussion of mass and weight.
A useful illustration of weight variations with respect to different
"frames of reference" is the fact that an earth orbiting astronaut is
nearly weightless in the reference frame of the spacecraft, but is not
weightless in the frame having its origin at the center of the earth. The
weight of the astronaut is reduced by only a small amount when in near
earth orbit wrt the earth. A larger reduction to about 1/6 the value on
earth wrt the earth arises in close orbit around the moon or on the
surface of the moon in the frame with its origin at the center of the
moon.
Gene Mechtly.
---- Original message ----
Date: Thu, 30 Aug 2007 08:16:48 -0400
From: "Stan Jakuba" <[EMAIL PROTECTED]>
Subject: [USMA:39292] Re: Current version of PCI list
To: "U.S. Metric Association" <[email protected]>
...
Attached please find the subject coverage as it is presented in my book
(pages 24 thru
31 if you have it).
