Re: a fan of "conserved-quantities first"

["John S. Denker" <jsd@MONMOUTH.COM>]

Chuck Britton wrote:
> I have been a fan of 'momentum first' for a decade or so now.
>
> We USE the momentum first sequence for the intro course that is LESS
> mathematical that the AP-B course.

A) Thanks for the note.

B) I tried something similar in an Ivy-league
physics-for-nonmajors course.

The sequence went something like this:

0) Remedial math.  Many of the students initially couldn't
solve one linear equation in one unknown.

1) Scaling laws.  Balloon twice as big has four times as
much surface area.

2) Conservation of "stuff".  Conservation of paint from a
spray-gun.  Twice as far away ==> four times as much area
(by scaling) ==> 1/4 the thickness (by conservation).

Application to photography (light intensity versus distance).

Quiz question:  Duck bobs up and down in a pond, radiating
waves that are confined to the surface (D=2).  Tell me the
scaling law for wave-height versus distance.

3) Conservation of energy.
 -- PE = constant: Lever (unequal arms / unequal masses).
 -- KE + PE = constant:  Pendulum; Roller-coaster.

4) Conservation of momentum, as JM and CB have advocated.

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

Analysis:  Some of the students liked it, some of 'em didn't.

The teaching-quality feedback reports from happy students
were all alike, but each unhappy student was unhappy in his
or her own way.  (http://www.bartleby.com/316/101.html)

 -- One was incensed that I would do anything unconventional,
because it impaired his ability to study from the "homework
files" and "exam files" accumulated over the years by the frat
house.

 -- One reported spending 1/2 hour per week on the course,
and judged that to be an unreasonable, onerous, oppressive
workload.  (This course had the reputation of being the
easiest way for non-majors to fulfill a "distribution
requirement" so it attracted quite a few people who really
didn't want to be there.  Lots of motivational problems.)

 -- One said "I showed this quiz to my boyfriend who's a
senior in the engineering school, and he couldn't figure
it out."  I replied "maybe he should take the course and
learn something."  This was not the response she wanted.

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

The real question remains, was this better or worse than
the traditional "F=ma first" would have been?  Sorry, I
can't give a scientific to that.  Not enough data.  But
generally it seemed to work OK;  no obvious reasons for
doing it the other way.