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Re: [Phys-l] g...



According to NIST (http://physics.nist.gov/Pubs/SP811/sec08.html) - the closest we have in the US to an "official definition":


"In science and technology, the weight of a body in a particular reference frame is defined as the force that gives the body an acceleration equal to the local acceleration of free fall in that reference frame [6: ISO 31-3]. Thus the SI unit of the quantity weight defined in this way is the newton (N). When the reference frame is a celestial object, Earth for example, the weight of a body is commonly called the local force of gravity on the body.

Example: The local force of gravity on a copper sphere of mass 10 kg located on the surface of the Earth, which is its weight at that location, is approximately 98 N.

Note: The local force of gravity on a body, that is, its weight, consists of the resultant of all the gravitational forces acting on the body and the local centrifugal force due to the rotation of the celestial object. The effect of atmospheric buoyancy is usually excluded, and thus the weight of a body is generally the local force of gravity on the body in vacuum."



This makes it clear that
* "apparent" weight is "the" weight
* weight changes with different reference frames
* weight is NOT (G m(earth) m(object)) / r(earth)^2, but rather it is this quantity minus centripetal effects.

We can argue all we want about whether this is the best definition, but it IS the accepted, standard, official US definition. Wouldn't it make more sense to get behind a single defintion, rather than having each book and each instructor trying to present the definition that he/she thinks is most familiar/useful/intiutive/pedogogically sound?

Tim F