Re: apparent weight

[LUDWIK KOWALSKI <KOWALSKIL@alpha.montclair.edu>]

> Date: Thu, 19 Feb 1998 08:35:53 -0500 (EST)
> From: "Donald E. Simanek" <dsimanek@eagle.lhup.edu>
> Subject: Re: apparent weight
>
> > 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. ....

Nobody would argue that the geometrical concept of *a ray of light* is a 
useful model. The same goes for the model of magnetic poles. More examples 
are needed to illustrate the usefulness of the centrifugal force model, 
that is to identify situations in which it helps students to make correct
predictions. Only two examples were given in this thread, separation of 
particles in a centrifuge and the shape of a spinning planet.

Here is an example for the usefulnes of magnetic poles. Couple of days ago 
we were talking (three teachers) about the old oscillators. Plug it into
an 60 Hz outlet and the frequency is 120 Hz. Why? The answer was that
an electromagnet goes over one cycle every 1/60 sec. In that time N
becomes S then S becomes N. But both S and N attract the oscillating
piece of iron. Try explain this without using the idea of magnetic poles.

Some people think that explaining phenomena in terms of magnetic poles
is as heretical as in terms of cetrifugal forces. I am glad to find out
that not everybody agrees with this. By the way, the omission of the
four letter word of calorimetry was deliberate. I still think it stands
for a useful concept at my level of teaching.
                                                    Ludwik Kowalski