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

[John Denker <jsd@av8n.com>]

Way back on 05/17/2008 02:18 PM, I wrote in part:

> 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
...
> So, why mention this?  Three reasons:
...
>  b) We should add this to the endless list of possible student misconceptions.
>   Life is hard enough when they show up with no idea how to use units, but
>   it is so very much harder when they show up with entrenched diametrically
>   backwards notions.
>
>  c) Even if the students have never encountered this before, at some point
>   in the game (not too early) we should warn them that they might encounter
>   backwards usage here and there.


Just today I got a lesson in unit analysis.  I was called 
upon to modify some software.  In the code I found tons
and tons of statements of the following evil backwards form:
  grains_per_octave = 4;
  longs_per_zone = 92;
  bins_per_zone = longs_per_zone * bins_per_long;

This was also a lesson in humility, since the evil backwards
code was something *I* had written previously.  Ouch.

I'm happy to report it was super-easy to repair the code.
I changed it to:
// Note _mi_ means "... measured in units of ..."
  fineness = 4;         // grains per octave
  zone_mi_longs = 92;   // zone size, measure in longs
  zone_mi_bins = zone_mi_longs * long_mi_bins;    // zone size, measured in bins

I mention this because it reinforces my earlier statement
that diametrically backwards abuse of units is rather
deeply entrenched in our culture.

If *I* need to struggle to keep this straight, it's scary
to think how hard the students need to struggle.