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Re: force



Dewey Dykstra, Jr. says:

....
Teaching kinematics in the traditional way seems to have too little effect
on too many students concerning whether they notice change-in-motion and
can themselves in their own minds and thinking separate change-in-motion
from motion itself. Hence in my view it does little to argue whether or
not to teach kinematics in the traditional way *either* before or after
forces.



What would be your view on teaching forces first (through statics), so that
they can then be used as a new tool in the teaching of kinematics? I think
that was the original point.

--
--James McLean

I have no problem getting students to consider forces in static situations
before looking at kinematics, etc. I've done this in the past using an
approach based on Minstrell's (see his article in Jan 82 TPT on the 'at
rest' condition.) But, for all the reasons given in my longer note
yesterday, I don't see how the resulting notion would be a any more a tool
in teaching kinematics than the notions of forces with which they typically
come to our courses. One of the, if not THE, essential distinction between
what physicists are generally satisfied with and what the students come to
class with is that "forces *cause* change-in-motion" and they use something
more like "forces *cause* motion" when specifically asked to explain
different kinds of motion via forces when they come to class. This
distinction is essentially in how forces relate to motion.

I don't see how forces from statics can be used as a tool in the teaching
of kinematics, except to engage them in considering discrepancies between
their expectaions, based on their "forces *cause* motion" view, and what
happens in experiments. ...unless, that is, one attempts to TELL students
that their ideas about forces are wrong and instead their ideas should be
"such and such." Unfortunately, this latter demonstrably (in typical
physics instruction) has almost no effect on their actual understanding of
the physical world concerning forces and its only lasting effects on MOST
students are essentially harmful. Also, it is very clear that their
"street" notions concerning acceleration (acceleration is speeding up,
slowing down is *either* deceleration or negative acceleration, and there
can be no acceleration if the speed or velocity is zero) is NOT a mere
sub-set of the "scientists'" notion of acceleration. Acceleration is *just
not the same entity* for them as it is for "us."

As is implied in the Chapter in the Fosnot edited book, a consideration of
statics appears not to even yield a net force idea for many students, at
least not one which shows up when they move to consider forces on moving
objects. Maybe the problem is that they do not see "no motion" as merely a
case of "constant velocity." The fact that they do not generally see "no
motion" or "stopped" as such is also documented in the research literature,
I'm pretty sure, but I can't give a ref right off hand. Hence, they do not
seem generally to be inclined or see it natural to extrapolate things
learned about the static situation to the moving one. This seems to come,
looking back on thing, rather than when moving on to what they consider a
different situation.

Dewey


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Dewey I. Dykstra, Jr. Phone: (208)385-3105
Professor of Physics Dept: (208)385-3775
Department of Physics/SN318 Fax: (208)385-4330
Boise State University dykstrad@varney.idbsu.edu
1910 University Drive Boise Highlanders
Boise, ID 83725-1570 novice piper
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