Re: Units and Conversions

[John Clement <clement@HAL-PC.ORG>]

> Regarding John Clement's  comment:
>
> > As to weight vs mass, this is a difficult problem.  In general I
> think math
> > tends to ignore the difference and I have even seen chem. teachers who do
> > not distinguish between them.  The standard conversion tables
> quote 1 LB =
> > 454 g without making any distinction between them.  The fact that this is
> > only true at sea level on the earth is never mentioned (at the right
> > location).  Physics students have a great deal of difficulty
> with this, so
> > you might try telling them, and then just using one of them, say mass.
> > Whether or not you wish to introduce the slug is up to you.  Technically
> > when you go from LB to kg you are not just doing a conversion of
> units, so
> > it should be a separate topic.
>
> John's claims that a) a particular conversion factor between the
> (Avoirdupois) pound (lb) and the kilogram "is only true at sea level
> on earth", and b) that conversion between lb and kg is "not just
> doing a conversion of units" are contradicted by the *definition* of
> the Avoirdupois pound.  See NIST Special Publication 811, footnotes
> 23 & 24, http://physics.nist.gov/Pubs/SP811/ .  In the US the
> Avoirdupois pound is *defined* as exactly 0.45359237 kg of *mass*
> *not* force.  Contrary to the beliefs of numerous physics and
> engineering teachers, the pound is a unit of *mass* in the US.
> This is convenient for commerce since it is most useful to denominate
> many commodities in units of mass since what is being purchased in a
> commercial transaction of some commodity is a given *amount* of the
> *stuff* in question--not a given amount of supporting (or
> gravitational) force.

This is a very good argument in favor of completely abandoning the use of
the pound in all physics texts as a unit of force.  Not having looked into
the legal definitions of the pound, I was completely ignorant of this
particular definition.  After all what are standards committees for if they
can't decide the meanings of particular terms.

Of course this still brings up the problem of the vernacular use of
weightless which physicists have attempted to resolve in various fashions.
Since physics terminology is generally determined by consensus rather than
by standards committees we need to find terminology that helps the students
rather than confuse them.  When and if the US makes the final transition to
SI, part of this problem may be solved, or maybe not.  In Tina's case the
simple solution is to simply use the legal definitions.  If the problem of
the usage of the word weight seems to be distasteful, then just say mass all
the time.

I had also forgotten about the usage of the poundal.  While it brings back
memories, I had thought that the old terminology of gram mass vs gram weight
and pound mass ... had gone away.  With all of this confusing terminology it
is a wonder that NASA has not had more screw ups.

Since students have been trained for 17 years of life that weight and mass
are synonyms, it is not surprising that this confusion persists in physics
classes.  To avoid problems I am very careful to use the term gravitational
force along with the term weight.  I also use just SI units.  Students who
go on to become engineers can learn the old English units when the time
comes.

I looked at the Information Please Almanac and it does not mention
conversion for force, but does mention conversions between SI kg and pounds.
However its dictionary has an interesting entry guaranteed to confuse
students.
---------------------------------
weight, measure of the force of gravity on a body (see gravitation). Since
the weights of different bodies at the same location are proportional to
their masses, weight is often used as a measure of mass. However, the two
are not the same; mass is a measure of the amount of matter present in a
body and thus has the same value at different locations, and weight varies
depending upon the location of the body in the earth's gravitational field
(or the gravitational field of some other astronomical body). A given body
will have the same mass on the earth and on the moon, but its weight on the
moon will be only about 16% of the weight as measured on the earth. The
distinction between weight and mass is further confused by the use of the
same units to measure both—the pound, the gram, or the kilogram. One pound
of weight, or force, is the force necessary at a given location to
accelerate a one-pound mass at a rate equal to the acceleration of gravity
at that location (about 32 ft per sec per sec). Similar relationships hold
between the gram of force and the gram of mass and between the kilogram of
force and the kilogram of mass.
---------------------------------
Did anyone on the dictionary committee ever hear of the Newton?  Apparently
this confusion extends to other sources as the World Book Encyclopedia has a
similar article without mention of the Newton.  The encyclopedia article is
much better written.  However all of this is probably moot as I doubt any
science students would consult either of these sources.  Incidentally some
of the science articles in the World Book are excellent and could be used as
references.

Thanks to David for pointing out the important legal information.

John M. Clement
Houston, TX