John Gaughan said:Ranee Mueller said:A pound or a kg of anything weighs the same as a pound or
kg of another thing.Incorrect. Kilograms are measures of mass, not weight.
Pounds are measures of weight, not pounds. Here on Earth
we define them to be roughly the same, but even on
Earth, depending on the circumstances, they are not
always the same.
Gentlemen of the jury, Chicolini here may look like an
idiot, and sound like an idiot, but don't let that fool
you: He really is an idiot. Groucho Marx
First thing you need to learn is that "weight" is an
ambiguous word, one with several different meanings. So lets
talk about the units of mass and the units of force; weight
can be either mass or some particular kinds of force.
Here's the official guide of the U.S. national standards
laborabory, NIST Special Publication 811 (1995 ed.), _Guide
for the Use of the International System of Units (SI)_ by
Barry N. Taylor:
Thus the SI unit of the quantity weight used in this
sense is the kilogram (kg) and the verb "to weigh" means
"to determine the mass of" or "to have a mass of".
Examples: the child's weight is 23 kg the briefcase
weighs 6 kg Net wt. 227 g
Of course, the same section also discusses other definitions
of weight,
Thus the SI unit of the quantity weight defined in this
way is the newton (N).
Then it concludes with the excellent advice that "whenever
the word 'weight' is used, it should be made clear which
meaning is intended."
The second thing you should know is that pounds have always
been units of mass.
Furthermore, both the avoirdupois pound and the kilogram
have also spawned units of force of the same name, recent
alterations which, although they had been used to some
extent for over a century before then, were never well-
defined units before the 20th century.
The main difference between them is that the metric system
is still fully supported and updated, and the keepers of our
standards have been telling us for about 44 years to stop
using kilograms force. OTOH, nobody is ever going to bother
to tell us to stop using pounds force, without telling us to
give up pounds all together.
The pound force is such a recent invention that it is
uniquely identified by that name. Of all the hundreds of
different pounds used throughout history, only one of them
has spun off a force unit of the same name.
Look at the troy system of weights--that's what we call it,
isn't it? This is still used for the weight of precious
metals and some gems.
There is one major way in which the troy units of weight
differ from both the avoirdupois pounds and kilograms--there
is no troy pound force or troy ounce force, and there never
have been any troy pounds force or troy ounces force. The
troy units of weight are always units of mass.
Pounds had always been units of mass, but by the middle of
the 20th century, they had gotten to be slightly different
amounts of mass at various places throughout the world,
before a common definition was arrived at. ngs.noaa.govFRdoc59Open ↗
5442.pdf gssp.wva.netrefine.pdfOpen ↗
Announcement. Effective July 1, 1959, all calibrations in
the U.S. customary system of by the National Bureau of
Standards will continue to be based upon metric
measurement standards and except for the U.S. Coast and
Geodetic Survey as noted below, will be made in terms of
the following exact equivalences and appropriate
multiples and submultiples: 1 yard = 0.9144 1 pound
(avoirdupois) = 0.453 592 37 kilogram Currently, the
units defined by these same equivalences, which have been
designated as the International Yard and the
International Pound, respectively, will be used by the
National Standards Laboratories of Australia, Canada, New
Zealand, South Africa, and United Kingdom; thus there
will be brought about international accord on the yard
and pound by the English-speaking nations of the world,
in precise measurements involving these basic units.
--
Gene Nygaard
ourworld.compuserve.comGene NygaardOpen ↗ "It's
not the things you don't know what gets you into trouble.
"It's the things you do know that just ain't so."
Will Rogers