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Re: Centrifugal force



As physicists who have studied at the graduate level, we have taken a
variety of courses in relativity that give us this view of gravity as an
acceleration field. We also learn to extend this concept of acceleration
field to non-gravitational situations, giving us insight into problems
in mechanics that would be confusing or intractable otherwise.

However, in general physics, the purpose of many first semester topics
(such as projectile motion or circular motion) is to reinforce the
students understanding of Newton's laws. I think we do a disservice to
the students when we analyze simple circular motion from our advance
view of non-inertial forces and acceleration fields instead of simply
doing a free body analysis of the forces on the object and calculating
the resulting acceleration. For physics majors, there's plenty of time
to study non-inertial frames later. For anyone who is not going further,
there is the danger that they'll go away with a deep misunderstanding of
Newton's laws.

We have a number of biologists at our school studying the motion of
jellyfish. They all have a feeling that the inertia of the jellyfish is
a force. This surprised me at first. I thought to myself that - well,
they're only biologists so we can't expect more of them. I got involved
with helping them with the video analysis they were doing of the motions
and eventually started reading some of the literature in their field.
The shock was - the beliefs they had in a force of inertia was ingrained
in the literature. It appears that somewhere along the line, someone
took the equations that we as physicists would write for non-inertial
reference frames, and applied those to ordinary frames. They talk about
the inertial force of the jellyfish pushing aside the water and also how
the inertial force of the water carries the nutrients to where it is
need in the jellyfish. Luckily for them, the water acts like a highly
viscous medium for the tiny nutrient particles, so their conclusions
turn out to be more or less correct, despite the awful physics that's
being used.

That is why I try to keep my general physics classes as simple and
focused on Newton's laws as I possibly can. I not only avoid terms like
centrifugal forces, I actively avoid assigning reading in those sections
of the text that discuss non-inertial forces at all. Students, in their
first encounter with physics, don't have the capacity to mix Newton's
laws with non-inertial analyses. They end up using the fictitious forces
as simply a way of breaking the rules set forth by Newton's Laws so they
can get the right answer. It's rare when a student can accurately
explain on a conceptual level the reason why they used non-inertial
terms in a solution.

By the way, I do cover relativity in the second semester after
electronics, and I do cover non-inertial reference frames at that time -
they are useful and valid concepts if introduced after the students have
become secure in their Newtonian thinking - IMHO, dangerous otherwise.

Bob at PC

"John S. Denker" wrote:

On 11/18/2003 01:33 AM, John Clement wrote:

> 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.

Remember, the centrifugal field is as real as the
gravitational field. No more, no less.