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Re: Newton's 3rd Law



Pang-Chieh,

There IS a nice chunk of research into teaching N3 by bridging analogies
done by Jim Minstrell and then more by Charles Camp & John Clement,
_Preconceptions in Mechanics: Lessons dealing with student's conceptual
difficulties_ Kendall/Hunt: Dubuque, IA (1994). ISBN 0-8403-9071-8

Basically the idea is to draw a model of a crystalline solid as a set of
hard balls linked by tiny springs. These model atoms suspended by
electrostatic forces in a cyrstalline matrix (I have a picture in Lecture 7
from last semester -- see
http://bonsai.physics.purdue.edu/phys152/spring96/lecture7/L7.html
)

What this does is makes reaction forces less mysterious -- they are explained
by subtle deformations in apparantly hard (inflexible) objects which are linked
to our everyday experiences with springs, Hooke's Laws and things like
springy matresses. Minstrell is pretty famous for showing the flex in a
lab bench by bouncing light off it and watching the beam deflect on a nearby
wall as he walks on the bench. N3 is a "spring" behaviour in nature.

This model is neat because it can be a precursor for field theory, to
materials study and also helps eventually understand momentum transfer in
situations like Newton's Cradle (or retarded potential in E&M). It's
the best model I know of to explain N3 and I've never seen it used in a
commercial, mainstream text (quelle surprise!)

The Sokoloff/Thornton Interactive Lecture Demos are also a nice way to
examine N3 using force probes and carts. I have used these with some
success as well. The texts and standard curricula are usually just junk
for this topic, which is pretty central to mechanics.

Dan the opinionated

Dan MacIsaac, Assistant Professor of Physics and Astronomy, Northern AZ Univ
Visiting Asst Prof, Purdue Univ; Adjunct Faculty, Indiana Univ at Kokomo
NEW NET ADDRESSES: danmac@nau.edu http://www.phy.nau.edu/~danmac



Pang-Chieh Chou

Can anyone share ideas on teaching Newton's Third Law? What do you do in
labs, demo, lecture, simulation, worksheets, etc. when your students hit
this stumbling block (no pun intended) ?

Pang-Chieh Chou
Elkins High School
7007 Flat Bank Drive
Missouri City, TX 77459-6111

phone: (713) 261-7600
fax: (713) 261-8361
email: pchou@tenet.edu