Re: [Phys-l] the first law of motion

[Hugh Haskell <haskellh@verizon.net>]

At 10:31 PM -0700 11/2/11, John Denker wrote:
> The first law has huge historical significance, but that's quite
> a separate discussion.  There is no law that says pedagogy must
> recapitulate phylogeny.  That is, retracing history is rarely the
> best way to teach a subject or to solve practical problems.

All correct, especially the last sentence, but the huge historical 
significance of NFL is a reflection of the great intellectual 
difficulty beginning students have with accepting NFL, even after 
they presumably understand NSL. I have had classes that do perfectly 
fine on problems involving NSL, indicating at least a superficial 
understanding of its importance to classical mechanics, and yet, when 
we start to reinforce the first law by showing its logical connection 
to the second law, at least a minority of students will still want 
there to be a "tiny, unbalanced" force to keep an object moving.

It is very difficult for them to recognize that air drag and other 
friction forces will bring the object to rest eventually, and that 
the "tiny, unbalanced" for they so strongly want to include is only 
necessary to counter those difficult to visualize drag forces. Even 
when they "intellectually" understand that, there still is a strong 
impulse to add "just a little bit more" to keep it moving.

It isn't hard to realize just why it took almost 2000 years for the 
Arisotelean picture to be replaced by Galileo's and Newton's 
insights. It is still a really difficult step for today's beginning 
students--even the really good ones that I used to teach.

I found that it was often easier for them to accept NFL if I 
acknowledged up front how long the idea that a force was needed to 
maintain uniform linear motion was the "standard model," and how 
difficult the transition to the modern picture was. It seems that 
once they understood that this was a legitimately difficult 
transition, they were more willing to embrace it, and didn't have to 
think of themselves as stupid if they had problems with it.

Pedagogy doesn't need to recapitulate phylogeny but it can't 
trivialize it, either--at least not a first.

Hugh
--

Hugh Haskell
mailto:hugh@ieer.org
mailto:haskellh@verizon.net

It isn't easy being green.

		--Kermit Lagrenouille