Re: [Phys-l] Mass and Energy

[Ludwik Kowalski <kowalskil@mail.montclair.edu>]

On May 25, 2006, at 11:50 AM, John Mallinckrodt wrote:

> > >   -- Should molar entropy be measured in bits/particle or in
> > > joules/kelvin?
> > >   -- Should mass be measured in kg or GeV?
> > >
> > > It's really not worth starting a holy war over such issues.  Use
> > > whatever's convenient, and do the conversions when necessary.
> >
> > To avoid "such  issues" I tried to use SI units consistently in
> > teaching introductory physics courses.
>
> In my opinion the consistent use of SI units in introductory physics
> courses only exacerbates the problems students have with units,
> problems that they WILL face in the cruel world, not to mention our
> own introductory laboratories. (Does anyone really conduct
> introductory laboratories in such a way that students never have to
> face unit conversions??)  Far better to make sure that they
> understand that units are NEVER to be taken for granted and that
> unit  conversions are ALWAYS trivial--a little tedious perhaps, but
> always trivial.)
>
> Students in lab are constantly asking questions like, "Do I have to
> convert the length to centimeters?"  My standard response is
> "Certainly not.  Do so if you want to, but units always take care of
> themselves as long as you treat them with enough respect to let them
> do so."  When you don't do unit conversions BEFORE a calculation you
> may end up finding that some force is (2.3 +- 0.1) x 10^-5 W h/ft.
> There is nothing wrong with that EXCEPT that it may be hard to
> compare with the product of m and a that you found to be (352.5 +-
> 0.5) g and (75 +- 2) cm/s^2 respectively.  No problem, just do
> whatever conversions are required in order to allow the comparison.
>
> The fact that problems like this arise constantly in lab should be
> all the evidence we need to appreciate the fact that students will
> not be shielded from the need to perform unit conversions in the real
> world.  Thus, an important part of our job ought to be making sure
> that they face that need squarely and with the kind of confidence
> that can only be borne of their own appreciation that a respect for
> units is critically important and that unit conversions are both
> routine and trivial.

Some conversions are trivial (such as miles to km or calories to J) 
others are less trivial (such capacitance in cm into F or calories into 
kg). Using a system of units in which speed is dimensionless (and c=1) 
can only confuse those who take a first physics course. That is why I 
decided to use SI units in all calculations. Yes, express everything in 
SI, if necessary, before performing calculations. The results can also 
be converted to other units, if necessary. In my opinion other systems 
of units should not be discussed in a first physics course. More 
advanced courses are better for this.

By the way, long time ago I wrote a paper about SI units. The title was 
" A Short History of SI Units in Electricity  ." The paper was 
published in 1986, in The Physics Teacher. You can see it at my old 
website:

http://alpha.montclair.edu/~kowalskiL/SI/SI_PAGE.HTML

I do not think that SI is the best possible system of units. But it was 
"officially chosen;" it is used in nearly all introductory textbooks, 
all over the world. That is a significant accomplishment.

Ludwik Kowalski
Let the perfect not be the enemy of the good.