Re: SI and nothing else.

[David Dockstader <DRDOCK00@UKCC.UKY.EDU>]

On Fri, 19 Sep 1997 23:04:29 EDT LUDWIK KOWALSKI said:
> Kilogram is now officially defined in terms of the mass of C-14.
Why was C-14 chosen instead of C-12?

> Why bother to talk about speed of light, ions, isotopes and atomic
> clocks too early?
I prefer to start with the historical definitions of the SI units.  There are
two advantages to this.  One is that the students can understand them, and
two is that the current definition is subject to change, but the original
definition is not.  I also think that a review of the history helps students
see just how arbitrary the definitions are.  It quickly becomes clear that
the really compelling reason for using any units is because they are the units
others are using and therefore they are the units that are recognized and
understood.  It also becomes clear that units must be defined in such a way
that they can be readily determined to the needed accuracy, and that this
requirement means that our definitions must be continuely refined to keep up
with changes in technology.
>
It was also written:
> > It seems to me that unless there is a *clear* and *compelling* advantage
> > to the use of a non-SI unit and only in a class intended for non-physical
> > science people, that the SI be used exclusively -- no matter what the
> > instructor is more *comfortable* with -- let the instructor get off his/her
> > mental rear end and be professional about it.
How can anyone disagree with this?  But can we ever agree on what is clear
and compelling?  Some have suggested that wanting the students to understand
what is going on is pretty compelling.  I have my students figure out how
many flights of stairs they have to run up to use the energy in a can of coke
and a candy bar.  Since somebody else has already labeled these products
I pretty much have to recognize their units.  On the other hand, I started out
thinking about light in Angstroms, went with SI and learned to think in nm
only to wind up with an Astronomy book that proudly states that Astronomers
do not stick to SI and it uses Angstroms.  Outside of school I rarely encounter
an SI unit and when I do the unit rarely has any significance or meaning to me.
I buy soft drinks in 1l bottles, but I never think of it as a liter.  I have
camera lenses with focal lengths marked in mm, but I never think of them in
mm, rather I think of them in terms of their field of view.  I can usually
pick out the right metric wrench to fit a nut, but for all I care a 13mm
wrench could be a #13 wrench.  I certainly never think about 1mm and then
imagine what 13 of them would look like.  If I want my students to have any
feel for the size of the earth, the thickness of the lithosphere or whatever
I'll always use miles, but if I talk about density I always use cgs since
I always think of everything relative to water anyway.  But I also make them
work problems with cubic yards, because that's how sand, gravel, and concrete
are sold....  Face it folks, what we teach isn't worth squat if the students
never take it out of the classroom, and outside of the classroom we don't get
to make the rules!
> > This philosophy pervades many of the threads on this list. We often hear
> > things like "Well you are correct (strictly), but why can't I do it my
> > old fashioned (read 'incorrect') way none-the-less."  This is often the
> > same as saying "I am too lazy to improve my teaching."
> >                                                            Jim Green
>     I am trying to be professional. And my question is:
>        Does sticking to SI always improve teaching?
>
> My personal preference is to begin with calories and switch to joules
> later. This approach protects me from explaining today's topics in terms
> of what they will learn tomorrow. "Borrowing from the future" should be
> kept to a minimum. I know how to do this while dealing with units of
> internal energy.