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[Phys-L] Re: A Third law question



First I wrote:

> Let me reiterate what John D. said. Conservation of momentum is the
best way I know of to get students to understand forces and their
affects, including N3. It doesn't require fancy equipment to get
students to accept it, only a few demonstrations and some hands-on
work by the students. If these are gen-ed students, they really need
to get that hands-on work, so they will know what to do with their
> students in due course, and why.
>
And then,

At 19:06 -0500 10/12/05, John Clement wrote:

And what evidence is there for this assertion? Can you show this by pre and
post-testing on either the FCI or FMCE? I truly doubt it.

Conservation of momentum is weakly connected to forces in the minds of
students. Indeed getting them to make the connection is fairly difficult.
There are some excellent direct demonstrations of the effect of forces in
the Interactive Lecture Demonstrations by Thornton et al that work fairly
well and show high gain on the FMCE. Can you demonstrate similar gain by
using conservation of momentum labs?

If you can get similar gain then by all means publish the results so that
others can use the same method.

I don't have FCI or FMCE results to show for this since I don't use
either one (I am not yet convinced these test measure anything of
real value, but let's not get into that argument here), but I have
taught an intro course for the past 10 years in which we start with
momentum. The students establish the conservation of momentum
empirically, using air-track collisions. No use of the word force
happens until after the momentum section is completed, but once they
understand momentum, then examining the details of collisions allows
force to arise in a very natural manner as "what it is that causes
momentum to change." Once we have momentum as a concept and they have
more or less internalized it, it is quite possible to "derive" all
three of Newton's laws (we use a rather arm-waving technique, and
make no claims of rigor, either here or with the students), but it is
also made clear to the students that we could have started with
Newton and then "derived" the conservation of momentum. Only after
Newton's laws are in their hands to we delve at all deeply into the
nature of force, acceleration is also a late-comer to this program,
as are the kinematics equations.

My admittedly qualitative impressions, based on a comparison with
general student performance over the prior ten-year period is that
the students come away from this with a firmer grasp of classical
physics in general. They appreciate the conservation laws, and they
understand more about the nature of forces. Such feedback as I have
had from former students has generally been positive. They say that
the physics courses they had in college were easy for them, compared
to other students who had been exposed to physics in a more
traditional setting.

I would agree that, in a traditional setting, force and momentum
would likely be only weekly coupled, since they are typically taught
as essentially unrelated topics, so it isn't likely that most
students would see the connection in a first course. But if one
follows a pedagogically logical sequence, in which each new topic
follows naturally from the preceding ones, it is much easier for the
students to see the connections. I have had students complain about
why we even have them learn about forces, since momentum is a so much
easier way to solve the same problems. The idea that both force and
momentum could be used to solve many types of problems never occurred
to out students before we started doing momentum first.

Ruth Chabay and Bruce Sherwood are teaching engineering physics to
students at NC State using this method (momentum first), and that is
the format that their text, "Matter & Interactions" follows. Since
they are in a better position to conduct research, I suspect that
they may have the sort of data you requested. Why don't you ask them?

Our program was developed independently of them, but in my
conversations with them, they indicate their impressions to be much
the same as I have outlined above.

Hugh
--

Hugh Haskell
<mailto:haskell@ncssm.edu>
<mailto:hhaskell@mindspring.com>

(919) 467-7610

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