Re: Newton's 3rd law? was Re: inertial forces (definition)

[Robert Carlson <Raacc@AOL.COM>]

In a message dated 10/18/99 7:53:38 PM Central Daylight Time,
cparker@EMPOWERING.COM writes:

> I am surprised by this statement.  Would you please expand on this idea?  I
>  have been teaching my students that they attract the earth as the earth
>  attracts them.  If I am teaching wrong ideas I would at least like to know
>  why the idea is incorrect.

Cliff,

Physics is the study of nature.  If you teach your students that they attract
the earth as the earth attracts them, then you are in good company.  And I
quote from texts concerning their presentation of gravitation:

Physics, 4th Edition, Resnick, Halliday, Krane, page 346 Figure 3.  F12 = -
F21, the forces being and action-reaction pair.

Physics for Scientists and Engineers, 3rd Edition, Tipler, page 300.  The
force F21 exerted by m2 on m1 is the negative of F12, by Newton's third law.

Physics for Scientists and Engineers, 3rd Edition,Serway, page 360.  That is,
these forces form an action-reaction pair, and F12 = -F21.

Saying this, I concede that this model is ultimately wrong, as all our models
of nature are probably going to be wrong.  At least I hope this is true.  I
certainly hope we never reach the point when everything is predictable.

My suggestion is to be honest with your students.  The first day of every
semester, I start my courses by showing Solitaire on the computer.  I start
the discussion by asking students how they would discover the rules if they
hadn't played the game and couldn't use the help button.  Eventually this
leads to the idea that they must play the game to learn the rules.  I equate
this to the study of nature and finding her rules.  I then lead this
discussion to the point, we will be studying models that tell us something
about the rules of nature.  These models will have limitations.  If you
decide to continue your studies in physics, then you will see where some
models fail and see new models.  However, if a model works for an
application, then use it and don't use a sledge hammer to pound in a shingle
nail.

Bob Carlson