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

[Joel Rauber <Joel_Rauber@SDSTATE.EDU>]

Hugh,
I think you confused what John said,

John Mallinckrodt wrote:

> 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'm guessing what you are calling non-inertial forces are precisely the
"inertial" forces John refers to above.  John is categorizing forces in to
one of two types above; both of which should be in your FBD's; and
associated with l.h.s. of NewtonII.

type 1) "real" forces
type 2) "inertial" forces

Joel

> -----Original Message-----
>
> I'm with you in this one, John. At least when we talk about beginning
> students. Including non-intertial forces on a FBD gives rise to
> endless confusion among beginning students. If they learn how to draw
> FBDs with just the "real" forces (the left side of NSL) and then put
> the centripetal part on the right hand side, they will get the right
> answer every time. Once they understand the difference between the
> centripetal force and other forces, and are experienced problem
> solvers, they can do whatever works for them in the FBD, but in the
> beginning...
>
> Hugh
>
> > 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
>
>
> Hugh Haskell
> <mailto://hhaskell@mindspring.com>
>
> Let's face it. People use a Mac because they want to, Windows
> because they
> have to..
> ******************************************************