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

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Sun, 24 Oct 1999, Bob Sciamanda wrote:

> I sincerely feel that the problem with student mis-interpretations of
> F_c=mv^2/r would be eliminated if, after treating the special case of
> circular, constant speed motion the general case would be presented and
> hammered home with copious examples (most texts don't do this).  It should
> then become clear that a very useful case of the (oft' used) pair
>
> F_x=mA_x;  F_y=mA_y      is the pair
>
> F_t=mdv/dt;  F_c=mv^2/r;
>
> ... I can say that I have never experienced this misconception as even a
> minor epidemic problem. ...

I agree with what I see as the thrust of your comments here, but I think
it pays to use different terminology like "tangential and transverse" or
"parallel and perpendicular" as someone else pointed out a few days ago.
"Centripetal" implies the existence of a "center" that is far from obvious
in the general case.  The term is just too loaded and too often abused for
my taste.  I can only accept your statement that you've never encountered
any major conceptual difficulties associated with the term; but I know
that I have.

I will point out, however, that in the fully three dimensional case, this
approach might become a bit less compelling as the precise direction of
the transverse or perpendicular component of F can become far less
obvious.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm