Re: force - with an assumption made explicit

["Dewey Dykstra, Jr." <dykstrad@varney.idbsu.edu>]

After sending the note below I realized that it *might* help if I made
explicit one "assumption" on which the ideas in it rest.  The "assumption"
is this:

A person's understanding of a phenomenon or of an explanation given of that
phenomenon is made up of *their* ideas/notions/beliefs concerning that
phenomenon or that explanation of the phenomenon.  This particular
understanding is a function of that person and has no independent existence
from that person.  If a person's (student's) understanding is the object we
wish to influence, then it makes sense (to me anyway) that we attempt to
understand that particular understanding and we attempt to understand why,
how and under what circumstances a person (student) is likely to change an
understanding they already have.

The portion of Roger Haar's note to which I responded:
> It seems to me that force is far more observable than is acceleration. I
> experience and feel forces--run into a wall or sit in a car and have it
> speed up, for example. Yes, there are accelerations involved, but what I
> feel are forces on me. These seem to me to directly observable--very
> tangable.
>
> Here's a problem. Drop a ball on a coil spring. At what point will the
> acceleration on the ball be zero? Most students and many physicists will
> say that it is when the spring is fully compressed. WRONG! It is when
> the net force on the ball is zero. At that instant the ball is still
> moving downward and compressing the spring.
>
> I think there might be some advantage, therefore, to teaching forces
> first then kinematics. However, a better approach might be to integrate
> them more than is presently done.
>
> Roger

Another thought in response to Roger's note:
It strikes me that Roger's example here does not necessarily demonstrate
that students need to know forces before they can study kinematics.
Instead the example illustrates 1) how people think forces and motion are
*related*, regardless of what they think of forces and/or kinematics and 2)
people's notions about acceleration, itself!  The example as well as the
wealth of data in the research in physics learning literature is that when
given the chance, people are most likely to relate force to motion and not
to change-in-motion.  (John Clement has an article way back in 1982 in AJP
which talks about this.  Many comments to the same effect have occurred
since.)  Hence, the association no force or balanced forces with no motion.
Additionally, it is thoroughly documented that a typical belief about
motion is that if acceleration is about speeding up (street version of
acceleration) or about any change in velocity (an intermediate stage of
thinking about acceleration, closer to the 'physicists'' notion) then an
object with zero velocity cannot have anything other than zero
acceleration; i.e., if it has no velocity, how can its velocity change?
(This is routinely encountered by those teaching students using the Tools
for Scientific Thinking materials from Tufts.  I believe that this is
mentioned in articles such as those from Trowbridge and McDermott and
others from the U of WA group on kinematics since about 1979 or so in AJP)
Couple this with the additional research evidence, that people tend *not to
even address* change-in-motion when they are asked to describe motion
(manuscript by me and Dale Sweet submitted to the International Journal of
Science Education, status: in review), and one has to wonder how to make
sense of the proposition of "teaching about forces first" in the
expectation that how *students* *understand* forces and motion will change
in the example cited.

As I indicated in a previous note, there are several ways to teach
kinematics.  One should not conclude from the previous paragraph that I
advocate teaching kinematics in the traditional way, but I do think and
have found in my own experience (as assessed by pre and post tests of
student notions) that directing students' attention to motion and its
description without worrying about explanation of that motion with the goal
of confronting details about change-in-motion helps.  The optimum method
that I have seen, which appears to violate many of the traditional 'givens'
about teaching kinematics, focuses on a conceptual basis first.  I use
materials adapted somewhat from the Tools for Scientific Thinking Projects
at Tufts and follow that with investigations on force based on Jim
Minstrell's work (for a description see article in Nov '92, Science
Education by Dykstra, Boyle and Monarch, the appendix in particular and
Chapter 12 in Constructivism: Theory, Perspective and Practice, Fosnot
(ed), Teachers College Pr, 1996).

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.

The rest of my original response follows:
Why don't "we" ever consider where the students 'heads' are with respect to
forces (including what *they* think forces relate to and how and why) first
and then follow Maurice Barnhill's advice which follows?

> Now granted, all the things I had done before *had gotten them to the
> point that they had two sets of discordant explanations, each of which
> they thought ought to be true.*  THIS CONDITION IS WHAT IS REALLY NEEDED
> IN ORDER TO MAKE CONCEPTUAL PROGRESS.

[added emphasis is my own]

"We" seem to be so wrapped up in "what" or "where" the students are
supposed to get that the process of how and why THEY might change the sense
they make of the world is totally ignored.  If this is the case then why do
we expect any results other than those "we" get regardless of what order we
"teach" the topics?

What *is* our justification for determining "what" or "where" students must
get?  ...and who does it serve?

Dewey

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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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