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# Re: SI Metric

• To: seaint(--nospam--at)seaint.org
• Subject: Re: SI Metric
• Date: Sat, 31 Dec 2005 19:11:18 -0500

```> From: Christopher Wright <chrisw(--nospam--at)skypoint.com>

> > The biggest problems:
> > 1) when is a pound a mass (lb) or a force (lbf) (e.g. how many slugs
> > in a kg) and what exactly IS a kip (e.g. kilo-lb or kilo-lbf), and

> To be candid, I don't have a lot of patience with engineers who don't=20
> understand the difference between mass and force. This is basic physics=20=
> that you learn in highschool and re-learn as a sophomore in every=20

> 1. Newton's laws are dimensionally homogeneous: both sides of the=20
> 2. Weight is a force--the force of gravity acting on a mass, so it=20
> 3. Mass is the quantity of matter--the number of protons, neutrons and=20=
> 4. Pounds are always forces. Always.
> 5. Kilograms are masses. Always.

The other thing that we all learned was that different devices are
required to measure force and mass - springs vs balance scales. Outside
of the science room, nobody cares much. As you note, 1/4 pound of
hamburger looks a lot like 0.113 kg of ground beef measured at
sea-level, regardless of whether the dimension is force or mass. Most
grocers are measuring product by force rather than mass regardless of
the dimension of commerce. The error is trivial but the application is
practical.

You will find a LOT of people who will argue either side of the issue of
"weight" as force or mass. Both sides of the argument are based on
traditional application in different areas of use (e.g. commerce vs
science). Complicating this are the "other" measurement conventions that
use kg as force, simply because of common use carry-over.

There's really three issues that I see:
1) Social convention; US governments will avoid forcing social
compliance (e.g. AccuWeather must use centigrade).
2) Commerce; is flexible and enforceable (e.g. the guy with the cash
will determine how the product is measured and the government will
decree methods of uniform measure).
3) Science; will use the units that make sense for the application and
this has no public, commercial or regulatory influence.

Engineers have to step across the boundaries for all of these, and, as
you note, be capable of relating mass and force. it remains that the
biggest problem is still pound-force & pound-mass and simply knowing how
it is used in any particular application without having to qualify it
(e.g. lbf vs lb).

In conversion tables, a pound is ALWAYS converted as mass units but a
kip is ALWAYS converted as force units.

And a 5 TON HVAC unit weighs 600 lb ....

--
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International

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