Re: [Phys-l] Mass and Energy

[John Mallinckrodt <ajm@csupomona.edu>]

> >   -- 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.


John "Slo" Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://www.csupomona.edu/~hsleff/OoPs.html>