Re: Newton's 3rd Law

[gregm@magi.com (Greg Marshall)]

> 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

*Two spring scales (push-pull if you can find them). Get two kids to push
or pull against each other and compare readings.
*Two force transducers from Vernier and MacMotion software (or windows
equivalent).. same trick, and look at the numbers as a digital display, or
note the shape of the F-t graphs as the kids play
*Put a mirror on a wall and bounce a modulated laser off it to a detector.
Send a tone over the beam and listen to the changes in the tone as a kid
pushes on an "immovable" object
*Do anything you can to suppress the 'action-reaction' statement of the
law, and always express it in the form "Forces occur in pairs, opposite in
direction and equal in magnitude, and always ON DIFFERENT OBJECTS". I
usually tell kids that the action-reaction statement results from the
evolution of language, that what Newton wrote was in latin, and what would
have translated as force in the 17th century is now to easily expressible
as something implying motion (one of my mechanics profs said something like
this, and it may even be true...) Care here will destroy the apparent
paradox of equal forces and accelerations.
*Suppress in your own lanquage the idea of "balance" or "unbalanced" forces
and replace it with the concept of "net force" of sum of all forces on a
single object as the determiner of its motion
*Get kids to express all of the laws in terms of formal statements, "An
object will continue...", own language or informal statements (get class
members to check mutually that these are complete and correct), situational
or exemplary definitions "When ...", and algebraic/vectorial formalism (a=0
iff Fnet =0) as well as lots of visual, tactile and psychomotor (?)
examples... kids gotta feel the physics!

If this is done well, then FBDs and a lot of kinematics and statics
problems get simple.

Sorry to be so preachy... a bit of a thing with me these days

Cheers
Greg Marshall
Coordinator, Math Science & Technology
Ottawa Board of Education