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



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..
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