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Re: SI Metric

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> From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>

> I think you just helped make Christopher's point.  There _IS_ a force-mass
> unit confusion in SI.  People mistakenly use kg (or other mass units)
> where they should be using N (or some other force units).  And you seem to

There may be confusion in John/Jane Q. Citizen's mind because lb is
given as lbm in conversion tables but we engineers use lbf. We know the
difference. I suspect that few people realized there was an error at the
air pump. The confusion is in the CONVERSION due to the dual application
of lb. In the application of SI, there is absolutely no confusion.

The point to be made is that the error was corrected on both the tires
AND the pump scale. As long as the units are consistent, the consumer is
bright enough to figure out that they need to match the numbers. It
would be a mess if the tires were in kPa and the gages were marked in
kg/m^2. In any event, to allow marketing the same product in certain
countries, the tire and pump companies are still including psf.

This is the Canadian experience. In other countries, they may not have
made a clean transition to SI and may still be using kg/m^2, in the same
way that they still use miles in Britain (a work in progress).

> I think that Christopher's point is valid.  The comon confusion among many
> (mostly general public) is on both sides.  On the US Customery units side,
> people are used to dealing with lbs for both weight and mass.  They are
> not aware that there is a difference.  For the SI/metric side, the general
> public are confused in the "opposite" direction...they consider kg as both
> a mass and weight unit.  So, I agree with Christopher that it matters

The general public will continue to use mass units for all commerce so
the lb~N conversion is irrelevent. Very few people need to be concerned
with masses accelerated at any rate other than g.

NIST for one, refers to mass as weight. So there is no issue about
calling kg a weight. However, NIST has no jurisdiction on the moon, Mars
or in my office.

> the difference between mass and weight.  We as engineers usually (at least
> we SHOULD) do understand the difference.

No doubt.

> else such as an executive or a marketing person) is basically "forced"
> (pun intended) to bow to market pressures and put on the incorrect units.

This goes back to one of my three postulations: The person with the cash
determines the unit of measure.

I recall having a conversation with the president af a large US
manufacturing company several years ago. They have many
federal/state/military contracts. I asked how they were preparing for
the pending requirement to supply in metric. His face went white and he
turned to his assistant who nodded agreement. Words came out of his
mouth but, if I was an investor, I would have pulled my money.
Fortunately for them, the "deadlines" were (permanently?) delayed.   

I have seen bathroom scales with lb~kg and lb~N. I'm not sure which side
is winning that (imagine going from 150 lb to 670 N, overnight). The
mechanism is irrelevent and the inherent error is within the range of
"good enough" for the application. My doctor weighs me in N but I still
think in lb (lbm or lbf) at home.

My daughter, in primary school, doesn't know imperial. Recent
engineering grads have expressed difficulty with imperial projects -
their minds are working in base 10.

Those who need SI will become SI savvy. Those who don't, won't. it's
kind of like the obsolete ASD/LRFD debate.

Start looking at the labels on products at your grocery store. 

-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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