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

[Stan Greenspoon <sgreensp@CAPCOLLEGE.BC.CA>]

> > The problem with students, as I see it, is that they interpret
> > "centripetal force" as being one of those four fundamental forces of
> > nature, separate from the gravitational, electrical and nuclear forces.
>
> This is a problem that is exacerbated in the textbook I'm using,
> Hecht's "Algebra/Trig" version. Chapter 5 is titled "Centripetal
> Force and Gravity"!
>
> > The same is true for the centrifugal force.  In my class, I prefer that
> > students use "force directed toward the center" and "force directed away
> > from the center" for centripetal force and centrifugal force.  Then, when
> > I ask which force is directed toward the center of the motion, I am less
> > likely to get the "force directed toward the center" as the answer.
>
> That is the solution I would like to use. How do you protect
> your students from the bad influence of textbooks? The market
> demands that "centripetal force" be included in all textbooks.
> Tradition does not serve us well in this regard either.
>
> Leigh


  One textbook, "College Physics" by Serway and Faughn, has addressed this
issue correctly in the new 5th edition. Whereas the 4th edition had a
chapter section entitled "Centripetal Force", the corresponding section in
the 5th edition is called "Forces causing centripetal acceleration". It is
obvious that the authors have deliberately removed the term "centripetal
force" not only from the section heading but also entirely  from the text of
the book.
  Unfortunately, these authors follow the standard ("Halliday, Resnick,...
" and "Cutnell and Johnson" being notable exceptions)  and in my opinion
misguided, practice of discussing circular motion dynamics in the chapter on
the Universal Law of Gravitation. This chapter normally follows chapters on
work-energy and momentum.
I  believe the more appropriate practice,  which I follow in my teaching, is
to cover circular motion dynamics  when problems involving Newton's Laws are
done.
In the same vein, I believe that the textooks should discuss circular motion
kinematics, (includng, of course, centripetal acceleration) in the earlier
chapter on two-dimensional motion rather than in the much later "Circular
Motion-Gravitation " chapter.


Stan

Stanley Greenspoon       Tel.:  (604) 986-1911 Ext. 2439
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