Re: Centrifugal force

[John Clement <clement@HAL-PC.ORG>]

>  > I think that anyone who is trained to get students to associate
>  > all forces as being due to interactions, will then assert that
>  > centrifugal forces do not exist.
>
> By that logic, one would assert that gravitational
> forces do not exist.

Absolutely not.  There is an agent for the gravitational force, namely the
Earth.  The gravitational force is an interaction between the earth and
other objects.  In the case of the centrifugal pseudoforce there is no agent
in the interaction.

When you say that students know there is a centrifugal force, they can not
attribute it to an agent.  This is the BIG problem.  Students must first
learn to attribute all forces to interactions before they can learn to
properly analyze more complex situations.  By allowing in the centrifugal
force, immediately students will be encouraged to make up all kinds of
forces which truly do not exist such as inertia force, momentum force ... 

Essentially from the point of view of inertial non rotating frames the
centrifugal force doesn't exist, so we can deny its existence to the
students.  I would however, not say that it never exists to a student, but
rather that if they can't identify the agent of the interaction, then the
force does not exist.  For those that insist that it exists I would say that
the term is used in advanced mechanics and that it is called a pseudoforce,
but that in the model that we use for analyzing motion, it does not ever
exist.

Now you may not be dealing with students who make up fictitious forces, but
I do deal with them, and in some cases they are the majority of a class.
Similarly I have to deal with students who do not understand that the normal
force is always perpendicular to the surface, the string or tension forces
always pull along the string ...  Such students will immediately draw a
fictitious force if they have a term such as centripetal or centrifugal.
They will also draw the other "real" forces associated with agents.  Then
they are very puzzled when they are asked to name the centrifugal which is
pushing on the object.  Since many of them are concrete operational
thinkers, they do not understand that a contradiction may require them to
give up an idea such as the existence of a force in a particular problem.

Now it may be that all of your students do not suffer from these problems,
but I suspect that they do, and that these problems have been concealed.
Incidentally, the physics teachers who deny its existence may have never
taken an advanced mechanics course in which pseudoforces are analyzed.
However, they should be capable of analyzing situations in inertial non
rotating frames using NTNs laws.  Once they can do this, they are certainly
capable of understanding advanced mechanics. The real worry is that many
teachers are not even capable of doing such analysis.  In addition they do
not understand pedagogy so they do not understand why students have
difficulties and how to treat the difficulties.  This latter problem is
present in the majority of physics teachers including PhDs at universities.
Indeed I would say that the university teachers are probably less aware of
pedagogy than HS teachers, because they have usually had NO training it it
or in the psychology behind it. 

John M. Clement
Houston, TX