Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: apparent weight





Message text written by INTERNET:phys-l@atlantis.uwf.edu

In my opinion, no harm can result from explaining
certain things in terms of *centrifugal* forces, magnetic *poles*, light
*rays*, etc. Yes, I am sticking out my neck again. Later courses, if any,
may elaborate on limited utility of simple explanations and deal with
hidden nuances. <

There's a simple operational test for whether it's "safe" to give a
simpl(ified) explanation in introductory courses. Pose a situation to test
student understanding of the concept--a simple situation students can
clearly visualize. Or better yet, set it up as an experiment or
demonstration. If you can find a situation of this sort in which the
student's simple model or concept gives a wrong prediction, or fails to
give the right prediction, then that simple explanation or concept
shouldn't be used. We are not talking about "hidden nuances" here, or
subtle phenomena, or fine points of graduate-level theory. I am talking
about the real world of direct experience.

My favorite example is this. One often sees the "rope and picket-fence"
analogy to "explain" what happens when light passes through two polarizing
sheets. (See Hewitt.) How often this is done in physics classes! But how
often does the teacher then propose inserting a *third* polarizer between
two polarizing sheets with crossed axes. The third is to be inserted with
axis at 45 degrees to the others. Now, will students with their
"picket-fence" understanding of polarization predict the correct outcome?
90 percent won't, the other ten percent will suspect some sort of a trick
and abstain. In my view this tells us that we shouldn't use the
"picket-fence" model for describing polarization. It only "works" (sort
of, and not quantitatively) for the one case (two polarizers). What's the
use of that kind of a narrow "explanation"?

In most cases, you may need to get someone else to provide a
concept-challenging situation. When one thinks entirely within a
simpl(ified) explanation one tends *not* to even consider anything which
isn't embraced by that explanation. One is not likely to think of the
concept-destroying case, even if it is a case easily observed in everyday
life, or easily demostrated in class.

-- Donald

......................................................................
Dr. Donald E. Simanek Office: 717-893-2079
Professor of Physics FAX: 717-893-2048
Lock Haven University, Lock Haven, PA. 17745
dsimanek@eagle.lhup.edu http://www.lhup.edu/~dsimanek
......................................................................