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

[Bob Sciamanda <trebor@VELOCITY.NET>]

John,
I don't lay down any hard and fast rules about what diagrams the students
might draw to aid their problem solving.  If it helps them to include a
vector representing some a priori known projection of the net force,
that's fine with me  ... so long as they understand their own notation.

I only disagree with your "To which I, and I think most of us, shout,
"Nonsense."
It is not nonsense; it is another way of diagramming what is known and may
be helpful.

You are always a civilized, rational font of discussion;  there are others
who, of late, are beginning to taint this group with pontifical
excommunications.  Sorry if I over-reacted by using your mild outburst as
an occasion for venting.  I love Physics and rationality - I detest ad
hominem bickering and grand-standing.

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: Thursday, October 21, 1999 8:44 PM
Subject: Re: Newton's 3rd law? was Re: inertial forces (definition)


> 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