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...".
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.


>>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.