Re: Boeing and metrcication question


"Nick Mueller" <muellernick@xxxxxx> wrote in message
news:5l7o19F6iha0U1@xxxxxxxxxxxxxxxxxxxxx
Ed Huntress wrote:

The point is that the standard units don't necessarily relate *in whole
numbers* to the things we actually measure. Trying to be neat and tidy,
metrics sometimes shoots itself in its own foot.

The "problem" is the acceleration on earth (depending on *where* you
measure
it). It is 9.81m/s^2. Thus the factor of 0.1... to "convert" (it is *no*
conversion) mass to force.
F = m * a

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 are: 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.



You sound like one of those pro-metrics folks who make up all of this
supposed neatness of the metric system, Nick, and then wonder how
everyone
else doesn't agree with you.

I don't wonder of anybody who doesn't agree but at the same time doesn't
understand the difference between mass and force.

I understand it quite well, thank you very much. The traditional units of
force have been defined quite precisely in terms that are perfectly
acceptable to the stickiest proponent of the SI. The latter just don't like
those derived units. They're inelegant. They're also very useful.


I only have to look at the domain-dependant units of pound, pondal, pound
force and whatever to see what mess it is.

I don't know anyone who uses pondals, and the pound, both as a unit of force
and as a unit of mass, is quite handy within its domain.

I am not domain-independent. I am not trying to write the General Theory of
Relativity. Neither are you, and neither is (almost) anyone else.


Read about the SI-system before you talk about it.

You really can be annoying at times, Nick. It's very unlikely that you've
read as much about the SI system as I have, unless you spent more than a
year, as I did, outlining a book on quality assurance.

--
Ed Huntress


.

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