Re: momentum before force

[jmclean@chem.ucsd.edu (James Mclean)]

Raacc@aol.com says:
> << Raacc@aol.com says:
> > ...
> > It seems to me that velocity should be introduced followed immediately by
> > momentum.  Then acceleration and net force can be defined as the slopes of
> > the velocity and momentum graphs respectively.)
> > ...
>
> As far as justification, I don't see why you need any.  If you were teaching
> someone English, and showed them an orange, you would say this is an orange.
> What would be your justification?  Why do we call an orange and orange?  Of
> course, you could go into the lingual history of the word, but is this
> important?  Physics has a terminology and momentum is one of the terms.  Why
> is momentum mv and not m^2v?  Because we don't define it to be m^2v, we
> define it to be mv.
>
> When introducing acceleration, I define it as the slope of a velocity versus
> time graph.  If momentum had already been introduced it would then be just as
> easy to define force as the slope of a momentum versus time graph.  From a
> graphical view, wouldn't discussing force before momentum be the same as
> discussing acceleration before velocity? 

I really like this idea of teaching momentum before force.

Of course you don't *need* a justification for momentum, if you are
willing to tell the students "this is/will be useful and important later;
trust me".   But I think you indicated previously that you like to organize
class so that the concepts introduced are all motivated.  (Which also seems
like a very good idea.)

So the question is, how do you motivate the students to focus on the
combination m*v, without reference to force?  The more I think about it,
the more I think the answer may be conservation of momentum.

In analogy, you don't need to explain *why* this thing is refered to by the
word "orange".  But if the class is on the nomenclature of animals, the
students will say "well, OK, it's an orange, but who cares."

Paul: as you say, the idea of momentum has an ancient history in which is
co-evolved with the idea of force.  I'm looking for a way to short-circuit
that evolution by motivating momentum *without* the idea of force ---
purely so that only one new concept needs to be introduced at a time.

-- 
--James McLean
jmclean@chem.ucsd.edu
post doc
UCSD