Curt Fischer <crf3@po.cwru.edu> wrote in message news:<3EE673E0.A6907CF9@po.cwru.edu>...
> Ken wrote:
> > 
> > Curt Fischer wrote:
> > >
> > > Brett Robson wrote:
> > >
> > >>>The last I heard, the Avoirdupois pound was defined as being the weight of
> > >>>27.7015 cubic inches of distilled water at 62 degrees F with the barometer
> > >>>being at 30 inches.
> > >>
> > >>1. avoirdupois pound is not an imperial pound
> > >
> > > You're right here.  Congratulations.  Don't let it go to your head
> > > though....
> > >
> > >>2. your definition is pound-force not pound
> > >
> > > A pound force is a pound.  So what are you talking about?
> > 
> > Well, Kevin's obsolete definition might have been better if he had said
> > *mass* instead of "*weight* of 27.7015 cubic inches...".
> 
> But since the pound has historically been a unit of force, 

Wrong. Pounds have always been units of mass.  Pounds force are a
recent spinoff, something that was never well defined before the 20th
century, and which even today don't have an official definition.

Only one of the hundreds of pounds used throughout history has given
rise to a force unit of the same name--that's how recent the history
of the pound force is.

> it is
> definitely better to speak of "weight".

That's a big part of your problem, a failure to understand that
"weight" is an ambiguous word, one with more than one meaning.  It is
a synonym for mass (in physics jargon) more often than it is a
particular kind of force in archery jargon for draw weight of a bow,
or a different kind of force (due to gravity) in physics jargon.

The troy units are units of weight--but they are always units of mass,
never units of force.

Weight means the very same thing as mass (in its physics jargon
meaning) whenever anyone talks about "net weight" or "troy weight" or
"carat weight" of a diamond or "dry weight" or "atomic weight" or
"molecular weight" and in many other instances.

> 
> > One might speak of weightlessness, e.g. in orbit.  It wouldn't occur to an
> > astronaut, however, to say that he/she's massless...  Astronauts composed
> > of non-baryonic matter, and hence travelling at light speed, would be an
> > interesting concept, indeed...
> > 
> > For those confused about these notions, the pound-force is the weight, or
> > *force* exerted by a pound of mass subjected to an acceleration of 1G; as
> > such, its expression in SI units would be in Newtons.
> > The pound, OTOH, is a unit of *mass*, and would be expressed in SI units
> > in Kilograms.
> 
> According to every technical book I have ever used, the pound is a unit
> of force.  

Pounds force exist.  Many technical books use them (and so do people
in their everyday lives, usually as part of a unit of some other
quantity such as pressure), but many of those which do also use pounds
as units of mass.  Furthermore, even the ones which don't use them as
units of mass usually mention pounds as units of mass; it is only in
the past couple of decades that you can run into a few  textbook
authors so poorly educated that they don't understand that pounds are
units of mass.

>Ergo, pounds measure weight.  

There's no "ergo" about it.  Pounds often measure weight when they are
units of mass.  Pounds force occasionally measure weight, but are most
often used for various types of force that are never called weight.

>Kilograms measure mass.  

True, with two big caveats.

When they are units of mass, they are units of weight:

NPL (the U.K. national standards laboratory) FAQ
http://www.npl.co.uk/force/faqs/forcemassdiffs.html

    Weight
    In the trading of goods, weight is taken to mean the 
    same as mass, and is measured in kilograms. Scientifically 
    however, it is normal to state that the weight of a 
    body is the gravitational force acting on it and hence 
    it should be measured in newtons, and this force 
    depends on the local acceleration due to gravity. 
    To add to the confusion, a weight (or weightpiece) 
    is a calibrated mass normally made from a dense 
    metal, and weighing is generally defined as a 
    process for determining the mass of an object.

    So, unfortunately, weight has three meanings 
    and care should always be taken to appreciate 
    which one is meant in a particular context.

NIST (the U.S. national standards laboratory) Special Publication 811
(1995 ed.), Guide for the Use of the International System of Units
(SI):

http://physics.nist.gov/Pubs/SP811/sec08.html

    In commercial and everyday use, and especially in common
    parlance, weight is usually used as a synonym for mass. 
    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

Second caveat:  kilograms force also exist, and used to be quite
acceptable.  They were officially endorsed by the CGPM in 1901 when
they adopted a "standard acceleration of gravity" for the purpose of
defining grams force, and were acceptable until the adoption of the
International System of Units (which doesn't include kilograms force)
in 1960.

We still see many vestiges of the use of this once-acceptable unit
today:  pressure gauges in kg/cm², torque wrenches in
"meter-kilograms" (the SI unit is newton-meters) are still readily
available, and thrust of rockets or jet engines (as they were most of
the time in the Russian space program into the late 1980s or early
1990s, or in Tom Clancy's nonfiction work _Airborne_, 1997).

>This
> is the reason that, in English (Sepponian) units, there is a no
> non-unity non-dimensionless constant called  gc (g sub c), which relates
> force to mass.  

So, in this dreamworld of yours, what exactly is the standard for that
pound?  What is its nature--something electrical, mechanical, or what?
 Exactly when (to the year is good enough, or a range of years if you
cannot do any better) was it made a standard, and by whom?  To whom
does that standard apply?  Who maintains this standard, and where is
it kept?

>Here's a quick review for those of you having
> difficulty:
> 
> http://gems.mines.edu/~mckinnon/DCGN209/Handouts/gc%20summary.pdf
> 
> Further evidence that the pound is a unit of force comes from
> established terms like "ft-lbs", which I hope everyone will agree is a
> unit of torque, not some mysterious unit of dimension (mass)x(length). 

Yes, it is ft·lbf for energy or work, and either ft·lbf or lbf·ft for
torque.  In earlier times, and in places outside North America,
foot-poundals were more common for these purposes than they are not.

American Society for Testing and Materials, Standard for Metric
Practice, E 380-79, ASTM 1979.

     3.4.1.4 The use of the same name for units of force
    and mass causes confusion.  When the non-SI units
    are used, a distinction should be made between force 
    and mass, for example, lbf to denote force in 
    gravimetric engineering units and lb for mass.

Of course, symbols for units of measure should also remain unchanged
in the plural--no language-specific "s" at the end, for example.

Now, you define for us a British thermal unit.  How much water? 
That's not the water which exerts a certain amount of force, is it?

What does it mean if a physicist says the latent heat of fusion of
water is 80 Btu/lb?  Or measures specific heat capacity in
Btu/(lb·°F)?  What are those units in the denominators?
 
> 
> You might also want to check out what a "slug" is.

Tit for tat.  You ought to check out what a poundal is.

This is the English unit of force in a system which is much older than
the one which contains slugs.

Slugs are a little used 20th century invention, which didn't appear in
engineering textbooks before 1920 or in physics textbooks before 1940.
 They exist only in one specific system (out of several such systems)
of mechanical.  After a brief heyday, mostly in the U.S. and Canada
(they weren't ever used much in any other places using English units),
they have pretty much disappeared again.

The system of mechanical units in which slugs exist is, like SI, a
"coherent" system of units, as that term is used in metrology jargon. 
That means that in the only system which has slugs, there are no pints
or gallons of any kind, not U.S. liquid nor U.S. dry nor imperial. 
There are no horsepower, no Btu, not even any psi because there are no
inches or miles or ounces.  These specialized systems of mechanical
units are only used only in calculations, and you often have to
convert into those systems before you can use them, or out of them at
the end because the units you want to use aren't in that system, or
both.

Poundals are the derived units of force in a different coherent
foot-pound-second system of mechanical units.  They are the force
which will accelerate the base unit of mass in this system at a rate
of 1 ft/s².

Fill in the blank:  the base unit of mass in this oldest fps system of
English mechanical units is the _____________.  (Hint: it is the "p"
in this fps system.)


> 
> > 
> > >>3. your definition is /less/ accurate than using a standard object
> > >>   (perhaps this would be an interesting homework project for you.
> > >>   Discuss a practical way of maintaining as constant the 3 variables,
> > >>   volume, temp, and atmospheric pressure. Be sure to mention the
> > >>   effect of measurement on values)
> > >
> > > Umm, why couldn't you use any number of commercially available devices
> > > to maintain the desired temperature, volume, and pressure?
> > 
> > First, because mass definitions involving a measurement of pressure
> > might be circular, as the standard pressure definition relies on mass,
> > acceleration and surface units... (Hint: how is the Pascal defined?)
> 
> Yes, but the definition under discussion here was not a definition of
> mass.  Ergo, no tautology.  

Wrong.  It was a definition of mass--you are just too dumb to
understand one of the words used in that definition.

>Your points about accuracy are well-taken; I
> do understand that there were reasons that the weights and measures
> people adopted the Pt-Ir standard.  But if 1893 levels of precision are
> what we're talking about, then the devices today would perform
> admirably.  

You cannot measure force today to anywhere near the accuracy which the
weights and measures experts could measure mass in 1893.

> Oh, I see, wait...Brent may have misinterpreted as serious Kevin's
> suggestion that we return to English units!  
> 
> > Second, because there's no practical way to measure and regulate the
> > volume, temperature and pressure with the required precision.
> > The Pt+Ir mass references have an estimated error in the 10**-9 to
> > 10**-8 range, which is the major reason these seemingly quaint objects
> > are still used, well, as mass references...
> 
> If we updated the definition to be the weight of a certain volume of
> liquid water at its triple point, wouldn't that take care of the
> temperature and pressure problem?  How hard is it to measure volume
> precisely?

How easy is it to maintain a whole pound of water at exactly that
triple point, all the way to the edge of the container?

A lot harder than it is to measure mass precisely.
> 
> > There's no better mass reference system known as of yet, even though
> > some new ideas are being investigated -- e.g. the Avogadro crystal
> > lattice approach based on accurately estimating the number of Si
> > atoms in a "perfect" sphere, whose dimensions are controlled by
> > interferometry.
> 
> What is the current uncertainty in Avogadro's number?  Why can't the
> standard unit of mass be the nucleus of one atom of carbon-12, which
> already has an atomic mass of 12 amu by definition.

Because your eyesight isn't good enough to count the number of those
atoms I use to balance a kilogram of hamburger--even if you had the
time and the ability to do the actual counting.

-- 
Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/
  "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