Re: [Phys-l] Centrifugal redux

[John Denker <jsd@av8n.com>]

On 03/18/2009 06:58 AM, Rick Tarara wrote:

> ....  The person swinging the bucket is in a non-accelerating frame
> whereas one can ride with the water in the accelerated frame.

Yes.

> Big confusion, I think, to mix the two.

That's for sure!


>  The intro student is going to relate to standing there swinging the bucket.

I wish that were true!  But it's diametrically not what I observe.


In the huuuuge majority of the intro students I see, there is a
tendency to identify with the rotating object ... so there will
be a rotating frame, complete with centrifugal forces, even if
this is not what the teacher wants.  (This may or may not be
mixed with a non-rotating frame.)

The tendency to spontaneously use rotating frames seems entirely 
understandable to me, for many reasons.

1) The students have experience with cars, bikes, swing-sets,
 park rides, et cetera where they are personally observing things
 in a non-inertial frame.  

 Even their experience swinging things in a circle often involves
 a rotating frame.  For example, if a big kid holds a smaller kid
 by the arms and gives them a ride, swinging them around and
 around, the big kid rotates.

 Their experience of swinging a bucket WHILE STANDING STILL, 
 passing the rope overhead as needed, is many orders of magnitude 
 less extensive.  This model is not the students' first choice.

2) Naive students have a tendency to blur the distinction between
 object and observer ... and the distinction between observer and
 self.  If the topic of discussion is a bucket going around in
 circles, they subconsciously ask, "What would happen if I were 
 going around in circles?"

 This is partly attributable to a wish for simplicity;  why carry
 around three concepts (object, observer, self) when they might be
 able to get away with only one?   Also there is the well known
 tendency of the not-quite-adult psyche to personalize everything.

3a) Usually the intro physics teacher doesn't want to mention centrifugal
 fields;
3b) with a few exceptions (e.g. Feynman) the physics book isn't 
 going to mention it, and
3c) the students aren't likely to read that part of the book anyway; 

 ... so unless the student brings it up, it the topic of centrifugal
 fields will be as dead as a dodo.  The fact that the subject enters 
 at all into intro physics classes tells me that students are dragging 
 it in.

4) Last but not least, many of these problems really are simpler in
 the rotating frame.
 -- Although the bystander gets a clean view of the motion, he does 
  not get a hands-on perception of the force.
 -- The situation in the rotating frame is static.  In contrast, in
  the bystander frame, the force is a complicated time-dependent vector 
  and the velocity is a complicated time-dependent vector.

 As is often the case, you must buy the tool before you can use it.
 That is, some investment is needed before you can quantitatively analyze 
 what's going on in the rotating frame.  (On the web, grossly wrong 
 derivations of the Coriolis effect far outnumber correct derivations, 
 AFAICT.)  But once you know how to use rotating reference frames, they 
 really do simplify some things.


===================

*) Students who have never taken physics know about centrifugal fields.
*) Non-physics teachers in the school know about centrifugal fields and
talk about them in front of students.  (Driver's ed, sports, etc.)
*) Adults on the street know about centrifugal fields.
*) Physics grad students know about centrifugal fields;  they are as
real as gravitational fields, i.e. they are consequences of using a
non-freely-falling reference frame.

So what is the point of forcing students to go through an intermediate
phase where they are taught centrifugal fields don't exist?  For all
of their life before class, after class, and north/south/east/west of
class, centrifugal fields exist.

IMHO it is a recipe for disaster to teach students things that cannot
possibly be true.  The students figure it out.  The teacher loses
credibility and loses respect.