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

[paul o johnson <pojhome@FLASH.NET>]

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 really did
> think that that was a fairly well ccepted 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.

Since I wrote the quote in question, I'll try to describe John's
misunderstanding. My message said that if an object is experiencing an
acceleration perpendicular to its velocity, there must be a force acting on it
that's causing such acceleration, and that force must be included in the free
object diagram. The student should be taught that such a force MUST be there
because of the object's curved motion. It's the student job to discover the
source of this force.

I feel that both my original statement and this paraphrase of it are quite
unambiguous, and both describe the way most people teach physics. Read it
carefully, John. I do not say that there is some ethereal force called
"centripetal force" acting on the object. What I DO say is there is some real
force (caused by some other real object) acting on the object, and it is this
real force that's causing its curved motion, and the name we give to this
radially-directed real force is centripetal force.

I think what John got hung up on was my calling this real force a "centripetal
force" right off the bat. I sympathize with him because my students also get
hung up on that name. For that reason,  I don't use the term "centripetal
force" in my lectures. Rather, I talk about a force (or a component of a force)
that's directed perpendicular to the object's velocity -- in the radial
direction, if you like.

poj