On Mon, 17 Sep 2007 13:23:28 -0400, "Ed Huntress"
wrote:


"Nick Mueller" wrote in message
...
Ed Huntress wrote:


Maybe you find a planet where a = 10 m/s^2. :-)

There probably is no such planet, which makes my point. The natural world
and natural phenomena do not succumb to attempts to make "rationalized"
multi-dimensional systems of measurement, most particularly systems that try
to build everything from a minimum (seven, in the case of the SI) number of
base units, which are themselves derived from only three fundamental units.

It works great in theory and doubtless it's an aid to many scientists
working in many fields. For others, including the field of medicine, where
I've been writing for the past few years, it simply results in a lot of
clumsy derived units. Thus, you'll see older CGS units mixed with SI units
in many fields, as a simple matter of practicality.



Nonsense. The Newton is defined in terms of kilograms, as well. It's just
that it's defined in terms of acceleration rather than as force itself.

You didn't understand the SI-system. It is based on **as** **few** **as**
**possible** units, the rest is derived/partially defined by them.
They a kg, s, K

Yes, from which the base units of the metre, the kilogram, the second, the
ampere, the kelvin, the mole, and the candela are defined. And then dozens
of other units are derived.

It's a theoretically elegent system. By using those base units, the SI
committee has developed a system that is theoretically coherent and
conceptually minimalist, but which also forces you to keep things in your
head that are far abstracted from what you're actually measuring, or to
memorize the system without thought -- which obviates any "rational"
advantage the system may have, in much practical use.

Again, we're not arguing over the advantages of the SI system to a scientist
performing elaborate calculations about celestial bodies and their
photometric properties, or remotely measuring their mass and angular
velocity. We're talking about the everyday measurements that make up the
vast majority of numerical evaluations made by people in the world. For
them, defining the unit of force in terms of acceleration, when they're
interested in how big they'll have to make a support to keep a cistern off
the ground, forces them to use (if they're using SI units), abstractions
that they'll have to memorize or convert roughly into something sensible --
the weight of that cistern when it's full of water. They're forced to use
Newtons, when what they're dealing with is kilograms of force, or pounds, if
they're so inclined.


snippage of noise and opinions :-)

Ed:-
I will be using Imperial measure until I die. Damnit, I will use Whitworth and
BSF threads in preference to others until I no longer have the capability to
make them. But.. The SI system holds together far better than the Imperial
system for anything that involves any form of calculation. I have no problem
with a 1 hp motor, but going from there to 550ftlb/S and 2,545Btu/hr
as opposed to a 1kW motor being 1000Nm/S and 1000J/S brings it home that the
SI system is _rational_

Other derived units:- 1F=1V/C=1Vs/A What's a Jar worth?
1Tesla=1W/m^2=1Vs/m^2 , I don't even know if there is an Imperial unit of
magnetic flux density!

The conversion factor between base and derived units is always 1, PI or E.
That really helps when checking a calculation for consistency. You _can_ do
the same with fps units, but only if you stick to base units. This isn't
abstract science, this is everyday engineering, Things like calculating the
impedance of a length of power line, or the size of the flywheel needed on a
press.

DEVIL'S ADVOCATE
The SI system will entirely replace the Imperial system and pretty much has
done in all of the world except for the USA and some scattered romantic
holdouts in the rest of the world. In 30-40 years time when the (federal) EU,
China and India all have larger economies than the USA, what will be the
advantage of using a measurement system that the rest of the world thinks is
mediaeval?
/DEVIL'S ADVOCATE



Mark Rand
RTFM