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# Re: SI Metric

• To: seaint(--nospam--at)seaint.org
• Subject: Re: SI Metric
• From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
• Date: Sun, 1 Jan 2006 13:43:27 -0500 (EST)

```Paul,

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
hand a perfect example.  You cited the tire pressure units on your tires
and the air pump for filling tires at the local gas station.  This would
be expressed in "pressure" units, which is a force unit per length unit
squared.  So, in your example, the lb/in^2 (psi) is 100% correct and there
is NO confusion on the US Customary units side, yet there is confusion on
the SI/metric side as you point out that they used to have kg/m^2, which
is wrong as that is a mass unit per length squard, that later got switched
to the correct units of Pa (or kPa to be more specific).  Thus, someone
was "confused" as to which SI/metirc units were to be used.

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
little which set of units are used as general public (and some engineers)
are confused with either.  I would argue, however, that it is not really
the system of units that confuses but rather that most don't understand
the difference between mass and weight.  We as engineers usually (at least
we SHOULD) do understand the difference.

That then presents us engineers with a problem.  Since we live in the real
world, we have to deal with the real world that does not understand such
differences.  And so, while we may know and see the incorrect use of the
units, we are kind of forced to use those incorrect type of units when we
interact with the real world both in our professional life or personal
life.  Are you gonna choose to not buy that deli meat cause they
incorrectly measure it in kg on a spring scale that is really gonna be
measuring a force (i.e. N or such)?  I am sure that the engineer that
designed that scale knows that putting a label or readout that measures in
kg was making use of incorrect units, but realized that if he/she put N as
the units, then no one would know what that hell that was and thus no one
would likely buy those scales to use.  The point is that the company that
makes that scale (whether it was the engineer at the company or someone
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.

Regards,

Scott

On Sun, 1 Jan 2006, Paul Ransom wrote:

> > From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
>
> > On Dec 31, 2005, at 6:11 PM, Paul Ransom wrote:
>
> > > Complicating this are the "other" measurement conventions that
> > > use kg as force, simply because of common use carry-over.
> > Which is my point--there's nothing about either system that's purer
> > scientifically. Both have the same built-in misunderstandings
>
> No. The force-mass unit confusion does not exist in the "SI" measure.
> Newton (force) is derived from the basic properties, mass (kg) and
> gravity. The standard mass is not determined using either spring or
> balance scales.
>
> > > In conversion tables, a pound is ALWAYS converted as mass units but a
> > > kip is ALWAYS converted as force units.
>
> > Pounds are more commonly used for force measurements--pressure in
> > lb/in^2; reactions in lb; gravity force in lb; engine thrust in lb;
> > moments in ft-lb etc. Anyone fool enough to do engineering analysis
> > using the same name for the unit of mass and the unit of force
> > distinguishes the two with the terms lbf and lbm. The reason kips are
> > spoken of as forces is because they _are_ forces, just like meters and
>
> For engineering applications, I agree on all points. Yet, we see gages
> marked lb/in^2 with kg/m^2 (or similar) because somebody found a
> conversion table. My tires and the gas station air pumps used to be
> marked this way (now kPa). Why? Because the average person is not an
> engineer and doesn't know a kPa but they believe that they "know" a
> pound and a kg.
>
> > kilometers are both lengths. Pounds are always force. When you say that
> > something weighs 1 lb, you're saying that the the something experiences
> > a gravity force of 1 lb. If you're talking about a mass of 1 pound
> > you're saying you have an object whose mass is such that the object
> > experiences a gravity force of 1 lb.
>
> My point is that not everybody agrees with this interpretation of pound
> or "weight". As a result, we have slugs (ptuuiii) and commercial
> measurement standards by goverment decree that define pounds as mass -
> not lb-force associated with a mass, etc.
>
> So, steel and concrete are sold on a mass basis (pcf and plf) but are
> applied on a force basis (pcf and plf), which have the same numeric
> values (gravity acceleration only) but very different meanings. It sure
>
> IBC2003: pound ~ N (force) as defined in the footnote to many tables.
>
> ASCE7: pound as force - converted values in text/tables only (no
> confusion). No conversion table.
>
> AISC: pound-force and pound-mass explicitly (ASD 9th Manual, conversion
> table). Section properties provided as plf which is commonly applied as
> force/length but is probably mass/length (see ASTM A6/A6M to confirm),
> despite the good practice in the conversion table.
>
> ACI: can't make up their mind(s)
> pound ~ N (force)
> lb/ft^3 ~ kg/m^3 (mass density)
> http://www.concrete.org/COMMITTEES/pdf/051024-Conversion%20Rules%20for%2
> 0ACI%20318.xls
> For static load applications, lb/ft^3 is commonly applied as a force
> "density".
> It is not clear if there is a conversion from lbf to lbm for ACI's
> applications or if they rely on engineers to make that little brain
> twiddle to accomodate.
>
> NIST: http://ts.nist.gov/ts/htdocs/200/202/mpo_home.htm
> "Mass (weight)" - pound ~ kg
> Force - "pound-force" ~ Newton (kg*m/sec^2)
> http://ts.nist.gov/ts/htdocs/200/202/fs376-b.pdf
> http://ts.nist.gov/ts/htdocs/200/202/forum/2005%20ForumReport.pdf
>
> Engineers REALLY want to see this:
> http://physics.nist.gov/Document/SIFedReg.pdf
> http://www.gsa.gov/gsa/cm_attachments/GSA_DOCUMENT/Metric_Design_Guide_R
> 2E-c-oW_0Z5RDZ-i34K-pR.pdf
> This document also discusses rebar sizes.
>
> It is interesting to see that it will soon be permissible, in the US, to
> label consumer products in SI, only.
> http://ts.nist.gov/ts/htdocs/200/202/forum/2005%20ForumReport.pdf
>
> Other links are available at US Metric Association:
> http://lamar.ColoState.EDU/~hillger/#metric
>
> You will have to stitch the multi-line URLs to get the correct links or
> cut them short and browse to find the reference.
> --
> R. Paul Ransom, P. Eng.
> Civil/Structural/Project/International
>
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