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[Phys-L] ? FCI --> momentum flow



Hi --

I have done some work on my document "introducuction to
force and momentum"
http://www.av8n.com/physics/force-intro.htm

New things:

I did some reorganizing. The document is still wildly disorganized,
but it's not as bad as it was.

I added a diagram and some discussion of my favorite demonstration
of momentum flow: Newton's cradle.

I added another example that illustrates the power and convenience
of the momentum-flow approach, namely an air-gun. There may be some
possibility of attention-grabbing demos :-).

The tension in a string is not a vector. If you rotate a vector 180
degrees, you get the opposite vector. If you rotate a string 180
degrees, everything looks the same, and you get the /same/ tension.

It really helps to have good symbols for things!
I cooked up a symbol to represent leftward flow of rightward momentum,
such as occurs when there is tension. Of course it is also possible
to represent leftward flow of rightward momentum, which is physically
the same. Meanwhile it is also possible to represent rightward flow
of rightward momentum, which corresponds to pressure (as opposed to
tension). Last but not least, there is downward flow of rightward
momentum, which is a form of shear.

============================

Here's where I'm coming from on this:

Typical FCI scores are an embarrassment. Any self-respecting physicist
looks at the questions and says "that's trivial" ... but then the students
get the wrong answers. In the literature people brag about "improved"
scores that are still terrible.

When I first looked at the FCI, I didn't see how anybody could get the
questions wrong ... but on second thought I realized that I could easily
get fooled by the force questions if I thought of them as force questions
... which I don't. I see them as momentum questions.

The force concept is great in simple situations. However, as soon as
there is any real complexity (e.g. fluid dynamics) or even any superficial
obfuscation (e.g. FCI), the momentum approach works better.

I have only limited facts on this, but even prior to gathering facts it
seems like a plausible hypothesis, worth testing. It might not work ...
but what we're doing now isn't working either, as evidenced by FCI scores,
so there's not much to lose.

The story goes like this: Students are already supposed to learn about
momentum in general and conservation in general ... so why not get them
to /use/ these ideas more? I'm not suggesting adding anything to the
syllabus or taking any extra time, just shifting things around a little
bit.

-- Rather than doing conservation of energy late in the year, do
it on Day One. Segue from there to conservation of momentum.

-- Shift from: third law as "equal and opposite reaction"
to: third law as "conservative flow of momentum"

-- Shift from: first law as "uniform motion in a straight line"
to: first law as "constant momentum" ... if momentum is
flowing out just as fast as it flows in.

-- Shift from: second law as "F=ma"
to: second law as "F = rate-of-change of momentum"
/and/ this is not circular or tautological because
we have a separate, operational definition of force.

You don't need to jump in to the deep end of the pool. You can shift a
little bit, and then a little bit more, and see how it goes.

Sometimes when I mention this I get pushback that says "the laws of physics
are complicated. The idea of flow is too simple. It's a third-grade idea.
You can't possibly explain the laws of physics in such simple terms.
Anything you teach about flow will have to be unlearned later."

I am delighted to get that kind of pushback, up to a point, because it
cues up my assertion that the laws of physics are not as complicated as
you think, I am not oversimplifying things, and nothing I have said about
momentum or conservative flow will ever have to be unlearned. The third-
grade notion of flow can perhaps be refined and extended, but it is not
wrong. For simple applications the momentum-flow approach is not worse,
and for complex, sophisticated applications it is better.

Again: What I am saying about the physics is simple, but not oversimplified.
That shoe is on the other foot. A great deal of what the typical introductory
text says about force is unduly complex and yet oversimplified, and will have
to be unlearned.

This is not a new idea. It has been tried. I don't understand why
it has not caught on more widely.

I quote from a chapter by Andrea A. diSessa
http://gse.berkeley.edu/people/andrea-disessa
"Principles for computer-based instruction in physics"
in the book: _Research on physics education_ vol. 156 (2004)
edited by Edward F. Redish, Matilde Vicentini, Società italiana di fisica

.... students taught by a momentum flow approach have little
difficulty with Newton's third law, which is a stark
contrast to typical instruction. Less difficulty with the
third law is because the law is reframed to a fair
triviality by momentum flow: If momentum is flowing from one
object to another, the amount of momentum lost by one object
is the negative of the momentum gained by the other; one's
loss is the other's gain.
....
Momentum flow works because students have relevant
intuitions about flow that can be built on, a prototypical
genetic epistemological consideration. Momentum flow seems
so odd to those who were taught the usual approach to force
and motion because they simply have not built on those
intuitions, and they cannot easily manage thinking in terms
of flow.

That tells me momentum-flow is one of those things that needs to
be explained one way to high-school students and explained another
way to high-school teachers. I don't object to that. Life would
be simpler if there was only one audience, but things are as they
are. One of the rules of effective teaching is "know thy audience"
... and teachers are a crucial part of the audience.

I reckon anybody who wants to write a textbook that is the least
bit modern needs to write /two/ books, book "A" for the students
and book "B" for the teachers. There's nothing easy about this,
but it needs to be done.