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

[Bob Sciamanda <trebor@VELOCITY.NET>]

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;

that the latter pair is a special case of the former pair with x/y chosen
to be parallel/perpendicular to the present velocity vector; and that the
latter pair is of universal applicability to general particle motion.

I can say that I have never experienced this misconception as even a minor
epidemic problem.  I don't think we teach the general case represented by
the latter pair directly, or forcefully enough.  Emphasize that both pairs
are just F=mA in component form - nothing more and nothing less.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Sunday, October 24, 1999 5:55 PM
Subject: Re: Newton's 3rd law? was Re: inertial forces (definition)


> . . .The confusion caused in the case of "Centripetal force = mv^2/r"
is, I think,
> pretty well acknowledged by most teachers.  . . .
> John Mallinckrodt