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

[Joseph Bellina <jbellina@SAINTMARYS.EDU>]

I'm getting into this late as usual, but I guess I would like to ask
why, having drawn a set of vectors to represent the forces acting on an
object that are real in the sense that there another object involved so
that N3 holds, that you would bother to resolve these forces into
components which are parallel to the velocity vector and perpendicular
to the velocity vector...and then to identify the net perpendicular
component with a special name?
I suggest there is little or no analytical value in doing this, rather
it may be a leftover from the discussion in which we convince students
that objects traveling with constant speed may be accelerating, and that
the quintessential case of the is uniform circular motion where the
speed is constant, but the object is continually accelerating.  In that
case the direction perpendicular to the velocity has special
meaning...and then, like our students, having learned a new trick we
want to use it everywhere.
Someone has probably said this, but I favor talking about radial and
tangential components of the acceleration, and considering the
centripetal force as a special case for special circumstances.

cheers,

joe

On
Fri, 22 Oct 1999, Robert A Cohen wrote:

> On Thu, 21 Oct 1999, John Mallinckrodt wrote:
>
> > I appreciate your point.  Indeed, my reaction (or overreaction, if you
> > will) was primarily to the original poster's apparent insistence that you
> > *had* to show the centripetal force on an FBD.  Perhaps I didn't make that
> > clear enough.  (And I still say it's nonsense!)
>
> I found Rick Tarara's post perfectly reasonable.  Perhaps it is because I
> interpret "centripetal" as "directed toward the center".  In other words,
> if an object is going in a circle, there must be a force directed toward
> the center.  The force diagram, naturally, should indicate a net force
> directed toward the center, should it not?  This net force might be the
> result of several forces or just one force, but it must be there just the
> same.
>
> The problem with students, as I see it, is that they interpret
> "centripetal force" as being one of those four fundamental forces of
> nature, separate from the gravitational, electrical and nuclear forces.
> The same is true for the centrifugal force.  In my class, I prefer that
> students use "force directed toward the center" and "force directed away
> from the center" for centripetal force and centrifugal force.  Then, when
> I ask which force is directed toward the center of the motion, I am less
> likely to get the "force directed toward the center" as the answer.
>
> P.S. I also thought Rick's explanation of the 3rd law was quite
> reasonable.  All too often, students think the normal force acting up on
> an object is the 3rd law counterpart to the gravitation force pulling down
> on the object.
>
> ----------------------------------------------------------
> | Robert Cohen               Department of Physics       |
> |                            East Stroudsburg University |
> | bbq@esu.edu                East Stroudsburg, PA  18301 |
> | http://www.esu.edu/~bbq/   (570) 422-3428              |
> ----------------------------------------------------------