Re: Centrifugal force

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

> Hear, hear! It is a common misconception that there is a "centripetal
> force" that must be included in a force diagram, and it is a huge
> source of confusion. We are much better off *never* using the term
> "centripetal force," and limiting the word centripetal to
> "centripetal acceleration." I have even seen seasoned high school
> physics teachers get bollixed by this. They know what the answer is
> supposed to be for, say, the force on an object at the top of a
> vertical circle, and then when they draw the force diagram, they
> include a centripetal force term and when their answer is nonsense,
> they recognize that it is clearly a wrong answer, but have no idea
> why it is wrong.
>
> I prefer to talk only about the "net force" on an object and then
> point out that this corresponds to the mass times the centripetal
> acceleration (in the case of UCM, of course).

When you even mention centripetal, students immediately start thinking about
centrifugal, so it is really better if both terms are avoided completely.
The Modeling program very strongly suggests that centripetal force not be
used, precisely because of the problem above.  I think instead one should
talk about the inward directed acceleration.  Students have already heard
the word centrifugal, and they easily confuse it with centripetal.

> The idea that centrifugal force "doesn't exist" is also widespread
> among HS physics teachers. That's probably better than having them
> believe that it is a real force, even in inertial reference frames,
> but their dogmatic assertion that it doesn't exist indicates to me
> that there are a lot of HS physics teachers out there who harbor some
> serious misconceptions about some fundamental aspects of Newtonian
> physics.

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.  The idea of a pseudoforce or perhaps a more modern word virtual
force is an advanced topic which is far beyond anything that should be
treated in the intro. mechanics courses.  So when talking with students I
would deny that there is a centrifugal force (a force that pushes you
outward when you turn a corner) because I wish students to firmly understand
the idea of interactions.  I would ask them to just draw and label all
forces.  I would classify a disbelief in centrifugal acceleration as a minor
misconception when compared to other concepts.  It was popular in some
physics books to label the reaction force to the centripetal force as the
centrifugal force.  In other words the force the string exerts on a whirling
mass is centripetal, but the force of the mass on the string is centrifugal.
I don't know if some books still do this.  Again, I would say this is ill
advised usage.

And yes, there are a number of HS teachers who harbor non Newtonian
conceptions.  I think that is an established fact.  For example I saw one
who thought that the point at which a spring stops stretching while
oscillating was the point at which the forces balance.  One only has to get
physics teachers to take the FCI or FMCE to find this out.  OTOH there are
grad students who have gotten poor scores on these tests.  The Modeling
program has pre and posttest scores for physics teachers on their web site.

John M. Clement
Houston, TX