Re: [Phys-L] ? FCI --> momentum flow

[John Denker <jsd@av8n.com>]

On 10/15/2013 07:32 AM, LaMontagne, Bob wrote:

> My personal feeling is that Newton's approach to force is much closer
> to a student's concept of force. His definition was that two forces
> are in the same ratio as the distances moved by an object starting
> from rest and acted upon by those forces for the same time interval.
> It is a direct connection between force and the visible distance an
> object is moved - very clear and intuitive.

I'm not going to debate "personal feelings".  De gustibus non
disputandum.  I will go no farther than to point out that some
other persons feel differently.

There are some non-personal questions that could be addressed,
starting with FCI scores.  If anybody has an approach that is
so "clear and intuitive" that students routinely score 100% on 
the FCI, or something close to that, then please please please
tell us the details.

I consider the FCI questions to be trivial physics underneath 
a layer of very mild obfuscation.

It is the common experience in this business that students may
/say/ the force concepts are "clear and intuitive" and they
may do fine on simple applications ... but as soon as there
is any real complexity (e.g. fluid dynamics) or even any mild
obfuscation (FCI) the train goes off the tracks.  The results
are an embarrassment to student and teacher alike.

My personal feeling is that
  a) For simple problems, the force concept and the momentum-
   flow concept work about equally well, and each serves as
   a check on the other, so folks really ought to learn both.
  b) For complicated problems, the momentum-flow approach is
   the only game in town.
  c) Teaching about momentum-flow incurs zero extra workload,
   because it is all supposed to be covered anyway;  the
   only difference is to move it from page 1000 of the text
   to page 1 (or maybe page 10).

============

Last but not least, the appeal to the authority of Newton is
unpersuasive in the extreme.  What he actually said was:

                       LEX II.
   Mutationem motus proportionalem esse vi motrici impressæ, 
   & fieri secundum lineam rectam qua vis illa imprimitur.

The word "momentum" did not exist at the time, but there's 
no doubt that his second law is talking about the change in 
momentum, as we can see from the definition:

                       Def. II.

   Quantitas motus est mensura ejusdem orta ex Velocitate 
   et quantitate Materiæ conjunctim.  Motus totius est 
   summa motuum in partibus singulis, adeoq; in corpore 
   duplo majore æquali cum Velocitate duplus est, et dupla 
   cum Velocitate quadruplus.

Reference:  http://www.gutenberg.org/files/28233/28233-h/28233-h.htm

============

Besides, what if Newton got it wrong?  I say that as a general
rule, should be given the best available evidence, not the most 
ancient evidence.

So, here we see two of the many problems with appeal to authority:
  a) It is verrrry common for the authorities to be misquoted, and
  b) We are not in thrall to the ancient authorities anyway.