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Re: [Phys-l] Centrifugal redux



I believe that this note could be summarized as a statement that discussion of centrifugal acceleration fields should be out of the scope of an introductory course for pedagogical reasons.

As an aside, it would be helpful to have had the website referred to included as a link in the post.

Joel R.

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement
Sent: Wednesday, March 18, 2009 12:42 AM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] Centrifugal redux

The reason for this posting was to illustrate that physicists can't seem to
agree on whether there is or is not centrifugal forces. The website
obviously is in the believing camp, but the website writer doesn't have some
basic understanding if you look carefully.

The research shows that students tend to treat "centripetal" force as if it
is a particular force, and do not realize that it is merely an
identification of the net force as being inward. Indeed students have a
great deal of difficulty with forces.

But one approach yields great benefits. That is the approach of considering
forces to be produced by interactions between physical objects. This
approach improves understanding of NTN3, and helps students avoid the
misconception that the Net Force is a particular force rather than being
just the sum of the various forces.

Once one has established forces as being interactions, there is no
centrifugal force because there is no interacting object that exerts it. So
in rotating frames the centrifugal and Coriolis forces must be labeled
something else. The term pseudo force has been used for this. These then
are modifications to NTN and not "real" forces. The shorthand centrifugal
force is understood by physicists, but causes problems with students.

The real problem with centrifugal force is that students and most people use
it to explain why water stays in a swinging pail even when the pail is
analyzed in an inertial frame. They will often identify this as being the
"force of inertia". This confirms their Aristotelian thinking that forces
are something you possess that keep you going.

While the concept of "centrifugal force" is useful in situations such as
flying a plane, it will interfere with acquisition of accurate physics
analysis in an inertial reference frame. Students the put in things like
inertial forces or centrifugal forces in a free body diagram drawn in an
inertial frame. Again, the solution is to get them to identify only forces
due to objects that are touching the analyzed object or the Earth. Of
course you do not need to understand physics to use many of its results. I
am sure that many electricians who are very competent do not consider that
the actual flow is electrons moving opposite to the conventional current
direction.

Also at the point of analyzing in an inertial frame, if a student asks, you
can say that the proper term with centrifugal is pseudoforce and that this
is used to help analyze problems in rotating frames. A better term might be
virtual force because students have a feeling for virtual reality.

The interaction model of forces is one of the best ways to help students
organize forces, but in that model centrifugal force becomes a convenient
analysis fiction. This really is understood by many textbook authors, but
they fail to realize this model in a coherent fashion. So the author of the
website is confused, and tries to prove the existence of centrifugal force
by calculations and appeals to authority. This illustrates how students can
have very hazy models for the physics but still succeed in doing many of the
calculations. However the poorly formed models will tend to retard future
progess. Incidentally some of the authority he quotes is obviously trying
to say that centrifugal force is a fiction, so the author of the website
does not understand what the authority is actually saying.

Oh, and all of the arguments so far have revolved around individual
convictions, and not about the website I referenced. I perceive the
individual who put up the website as being obsessed with the idea that
centrifugal force has to be "real", and is attempting to prove this
conviction. I did not find the attempts to be very convincing, but perhaps
others will. Reality is in the mind of the beholder.

John M. Clement
Houston, TX


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