dvdfan9@hotmail.com (Ken) wrote in message news:<c5fec6f.0306101354.2f37f36f@posting.google.com>...
> 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...".

It wouldn't have been anything different if he had done so.

A troy ounce is a unit of weight, isn't it?  But the units in the troy
system of weights are always units of mass, never units of force. 
There is no troy ounce force, never has been.

> 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.

So how much acceleration is that?

There is an official acceleration for the purpose of defining grams
force, adopted by the CGPM in 1901 for exactly that purpose (this is a
concept of metrology, not of physics, something which serves no
purpose other than to define units of force based on units of mass): 
980.665 cm/s².

But there is no official value for the definition of pounds force.

> The pound, OTOH, is a unit of *mass*, and would be expressed in SI units
> in Kilograms.
> 
> 
> >>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?)
> 
> 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...
> 
> 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.
> 
> 
> >>if you don't understand any of the above please refer to an introductory
> >>physics text.
> > 
> > Could you let me know which introductory physics text you referenced
> > before writing your post?
> 
> *Any* decent high-school physics text should have made clear the
> fundamental difference between weight -- e.g. pound-force -- and mass
> -- e.g. pound -- units...  Whether the difference will be understood
> by all the students is another matter, of course.

Few high school and fewer college textbooks use pounds at all any
more, even in the United States.

Furthermore, a few these textbooks which do mention pounds (even if
they rarely or never actually use them) are now being written by
people so poorly educated themselves that several of them claim that
pounds are not units of mass.  E.g., Halliday, Resnick, and Walker
(1997 and a later one too).  Of course, if you give a couple of
octagenarians an ego boost and a token royalty to keep their names on
the book, while turning it over to an entertainment columnist, with
editors more interested in pretty pictures and colored text than in
the written content, it's no surprise if what you end up with is a
farce.

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