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Re: [Phys-l] momentum dissipation?




The Newton's cradle apparatus offers an easy, clear
demonstration:
-- initial: ball #1 in motion, incoming;
balls #2,3,4,5 at rest.
-- final: ball #5 in motion, outgoing;
balls #1,2,3,4 at rest.

Momentum de-accumulates from ball #1 and accumulates in
ball #5. Momentum flows through balls #2,3,4 without
accumulating there.

This is profoundly important physics ... yet it is easy
to understand, easier than the force laws. I have no
trouble explaining this to third-graders.

Ahh, but do they understand it? The fact is that third graders (age 8+)
often have difficulty with understanding displacement of water, indicating
that they are often not ready for conservation reasoning. Conservation
reasoning should have clicked in by then, but it usually does not. Even in
the senior year of HS there are students who have still not grasped
conservation reasoning.

The true test of understanding is whether they can transfer the idea to
similar but different situations. This is what the FCI is designed to test.
So the ability to explain does not confer understanding on the part of the
recipient. This is the big result from PER and other science education
research.

Third graders really do use different reasoning than students at age 10+
according to the research. My daughter confirmed this when she was a
counselor for younger students and she said their reasoning was illogical.
She had crossed over to formal operational and the younger children were in
the early stages of concrete operational. Mature concrete operational (age
9) should be able to understand conservation, but a fair fraction of adults
never get there. Similarly 2/3 of the population never gets to formal
operational which can click in at age 10+. And probably 25% of students in
calculus based intro physics courses are still below the formal operational
level, with only a small fraction at full formal operational (understanding
up to statistical reasoning).

John M. Clement
Houston, TX