Stan, Position, speed, acceleration, force, and weight are *not defined* (do not exits) without a "frame of reference" (an origin and coordinates) either specified or implied. This *requirement* exists even in the most simple Classical Mechanics (relative motion (speeds) much less than c), long before Einstein Relativity (speeds approaching c or accelerating frames of reference) come into play.
These variable quantities (position, speed, acceleration, force, weight) *must* be defined before units for force and weight (as a force) can be specified. The reverse order; first units, and then quantities is *impossible* however diligent and well intended the effort. For example, I am thinking of UnitX, UnitY, and UnitZ. Now you tell us which variable quantity each of these units represents, and how each of these units is related to the corresponding SI Unit. It can't be done! Gene. ---- Original message ---- >Date: Wed, 19 Sep 2007 09:44:18 -0400 >From: "Stan Jakuba" <[EMAIL PROTECTED]> >Subject: Re: Mass and Weight in Jakuba Book >To: "U.S. Metric Association" <[email protected]>, <[EMAIL PROTECTED]> >Cc: "SCC14 IEEE" <[EMAIL PROTECTED]> > >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). >>> >> > >
