Re: [Phys-l] backwards units : entrenched usage

[Hugh Haskell <hhaskell@mindspring.com>]

At 14:18  -0700 5/17/08, John Denker wrote:
> The other day I was reading the data plate on an electric motor:
>
>        HP  :   1 1/3                Type : C
>        RPM :   1725                 SF   : 1.0
>        A   :   12.3                 PH   : 1
>        V   :   115                  Hz   : 60
>
> This seems backwards relative to the way we would write things in the
> physics lab:
>
>       1.333 HP
>       1725 RPM
>       12.3 A
>       115 V
>       60 Hz
>
> A Philadelphia lawyer might argue that the A and V on the data plate
> stand for amperage and voltage ... but that only goes so far.  I'm
> not going to buy horsepowerage or Hertzage or RPMage.
>
> I'm not a historian, but I conjecture that the idea of "unit analysis"
> (where the units are algebraic quantities, with their modern meaning)
> is relatively new.  I've seen the "backwards style" in old engineering
> books.  There's no doubt that the colons on the data plate are the
> equivalent of equals signs.  In old books I've seen formulas of the form
>
>         RPM = 28.75 * Hz
>
> which is utterly backwards from a modern unit-analysis point of view.
> We would write the conversion factor as 28.75 RPM per Hz.
I dunno. Maybe this is just a historical phenomenon, but it may also 
be a natural thing for people who are not well trained in science. As 
you point out, electricians seldom use the words current or 
potential, but they also seldom use any of the accepted phrases for 
rotational speed or power. I have heard them frequently say "the RPMs 
are . . ." and "the HP is . . ." And my students often used terms 
like that, substituting the unit for the name of the quantity--"The 
joules of this object are . . ." etc. Look on the label of a can of 
soda. It won't say Energy: xx joules, or yy Calories. It always says 
Calories: yy (I used to have a Coke can from Australia that listed 
energy content correctly, as xx joules).

I suspect that this is more due to sloppy teaching about units and 
the names of things in our schools. The person who designed that name 
plate (and most of the other name plate designers) probably had hand 
no more than a high school physics class if that, and the person who 
gave them the specs to include may either have known no better or, 
worse, was used to the common usage and didn't care that it was wrong.

This may have been common scientific or engineering usage many years 
ago, but I don't think today's students have seen these things enough 
to have absorbed that bit of history yet.

Students in the typical physics course are being inundated with new 
concepts and the units associated with them and it is probably 
difficult to sort out which are which. They can only absorb so much 
at a time and since the teachers probably emphasize the units more 
than the name, it may be natural for them to remember units before 
the do names.

I do, however, sometimes see unit conversion tables expressed in the 
form you showed, which as a computational algorithm makes some sense.

This is mostly conjecture, but based on my experience with my 
students, and my (admittedly fuzzy) memories of my student days.

Hugh
--

************************************************************
Hugh Haskell
<mailto:haskell@ncssm.edu>
<mailto:hhaskell@mindspring.com>

(919) 467-7610

Hard work often pays off after time. But Laziness always pays off now.

			February tagline on 2007 Demotivator's Calendar