Importance of physical inquiry (Was: Loss of KE)

[John Clement <clement@HAL-PC.ORG>]

> I'll just add one more thing to JD's comments.  Students can work through
> the microscopic detail forever and not come up with a good reason for the
> loss of exactly 1/2 the KE in the collision as originally described.  This
> seems to me a clear case of _needing_ to start with the principles and the
> equations (center of mass arguments being the most convincing to me) and
> THEN worry about how the KE actually gets dissipated (or whatever phrasing
> Jim Green would consider appropriate to describe the accounting
> change!  ;-)
>
> Rick

While it is certainly true that worrying about microscopic details will not
come up with the answers, it is equally true that starting with principles
and equations will turn students into equation hunters.  You _need_ to first
have them do experiments where they actually see the principles in action
before they have been told the principles.  Also, beginning physics students
will not comprehend doing the arguments in the center of mass.  That may be
clear to the seasoned physicist, but is very unclear to the  beginning
student.  Once the physical principles have been established some
microscopic considerations may help cement the understanding.  Anyone who
has done simple experiments measuring before and after velocities of
inelastic collisions with carts will be convinced that there is an
underlying physical principle independent of microscopic details.

Also long before this is done students need to have a good understanding of
NTNs 3rd law.  Experience with the FCI and FMCE is very convincing that
students do not gain a good understanding of this law.  The appropriate
physical experience is _vital_ to understanding.  A recent article by
Priscilla Laws in TPT shows that it is possible to push this understanding
up to the 90% level as measured on the FMCE.  Notice that according the
learning cycle approach the physical experience should precede the formal
presentation.

John M. Clement
Houston, TX

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.