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

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Thu, 21 Oct 1999, Bob Sciamanda wrote:

> Let's lighten up!
>
> John, I think you are being too harsh and impolite, or you mis-understand
> the man!

Bob,

Wow.  Sorry if I offended anyone.  Frankly, however, I am a little
surprised to hear that you apparently disagree with me.  I've always found
it infinitely preferable to have students include in their FBD's only
those "real"  and "inertial"  forces that will ultimately be accounted for
on the "left side" of Newton's second law and to use the kinematic
information to help with the acceleration on the "right side."  I really
did think that that was a fairly well accepted procedure.

Do you really counsel students to show on their free body (or object)
diagrams a "centripetal force" when the object is experiencing an
acceleration perpendicular to its velocity?

On the other hand, perhaps I did misunderstand something.  The quote I was
reacting too read:

> ... Students should be taught to use free object diagrams in both
> equilibrium and non-equilibrium conditions. Far from being gratuitous,
> the centripetal force (that is shown in the diagram ONLY when the
> object is experiencing an acceleration perpendicular to its velocity) is
> necessary to justify and explain the non-uniform motion. The student
> should be taught that the force MUST be there because of the motion;
> the task for the student, then, is to discover the source of this force.

That looks pretty unambiguous to me.

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