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Re: force



John Mallinckrodt wrote:

On Tue, 29 Oct 1996, Paul Camp wrote:

I believe that
consideration of forces should follow immediately upon that of
acceleration and precede things like projectile problems and circular
motion. It seems to me it would help in reasoning about the direction
of a to understand it as the result of an interaction and it
mystifies me why most textbooks do not do so immediately. But that's
just my opinion, for which I have no data in support.

I'd go a step farther than Paul is advocating. As Martha Takats wrote a
while ago, it seems to me as well that beginning a course with constant
acceleration kinematics is a strong contributor to the confusion about and
distaste for the subject that students often develop. Viewed from a
student's perspective, the subject seems to require "memorization" of the
largest and most complicated set of equations that they will see in the
entire course. Furthermore, even those many students who do clearly
distinguish the definitions of acceleration and velocity when asked to do
so, will continue to make unconscious mistakes that only exacerbate their
confusion about forces.

Accordingly, I have begun writing a text that returns to a practice that
was once fairly common--a thorough study of statics before any serious
look at accelerated motion. I would like students to come out of the
initial unit with a view of the world as a place filled primarily with
deformable objects that are interacting with each other, are not moving
relative to each other, have settled into their positions obeying only the
requirement that there be no net force or torque acting on them, and that
deform as required to achieve interactions of the necessary magnitude. We
will consider the lowly spring over and over as a model for how
deformation leads to increasing force, for how objects can
self-equilibrate, and to establish a restricted version of Newton's third
law.

This unit will also consider the behavior of objects in cars and airplanes
moving at constant velocity and the fact that the earth itself moves
through space (whatever that means!) to establish that SIMPLE motions do
not pose ANY additional complications and are in fact entirely a matter of
one's viewpoint (i.e., Galilean relativity.)

Only after this view (which will later facillitate the study of
oscillations about equilibrium) is established will we then look at the
VERY unusual case of objects for which these forces have become UNbalanced
and ask what happens to them. It should be evident that something more
complicated than constant velocity will result and, after all, what is the
simplest thing that is more complicated than constant velocity?

I'd be interested in any feedback that all of you might have on the
worthiness and potential pitfalls of such an approach.

John
----------------------------------------------------------------
A. John Mallinckrodt email: mallinckrodt@csupomona.edu
Professor of Physics voice: 909-869-4054
Cal Poly Pomona fax: 909-869-5090
Pomona, CA 91768 office: Building 8, Room 223
web: http://www.sci.csupomona.edu/~mallinckrodt/

I can't comment with any personal experience, but I have a colleague who
firmly believes that students don't fully understand when there will be
accelerations and in what direction until they fully understand the net
force acting. Note that I wrote NET force. Too often the book, and we as
teachers, just say "the force acting on the object" when we should
stress that it is the net force acting.

How would you feel about starting with the concept of force? Is there
anything sacrosant about starting with kinematics other than that is the
way beginning textbooks always start.

Roger