Re: [Phys-l] Coriolis effect puzzlement

[John Denker <jsd@av8n.com>]

On 12/03/2011 06:16 AM, Philip Keller wrote:
> I wasn't expecting it and I thought "What makes the car start to
> rotate?".  Then I realized that the car didn't START to rotate -- it
> was already "rotating" as it circled the track, just like the moon.

People seem to be having fun with this one.  It might be
worth doing it for real (not simulation) using an air-hockey
table or some such, and posting the video.

Let's explore the obvious /pedagogical/ question:  Why is it
not necessarily obvious that the car will spin after the rope
breaks?  Or ... Why is it not necessarily obvious that the car 
was already spinning?

Here's my conjecture:  Even though rotating frames are beyond the
scope of the usual introductory course, students (and others!)
have a strong tendency to use rotating frames anyway.

  -- In any situation involving a car, it is natural to imagine
   that you are *in* the car, observing things relative to a
   frame attached to the car.

  -- In any situation involving a rigid body, it is natural to
   analyze things relative to a frame attached to the body.

   In particular, it would be fun to do a variation of the
   experiment:  Use four cars, all tied together, arranged
   90 degrees apart on the circle, chasing each other around
   and around.  This creates an even stronger impression of
   "rigid body" rotation.  Then disconnect all four ropes at 
   once.  The cars will start to spin /relative/ to the
   rotating frame.   

There is nothing wrong with using a rotating reference frame, 
if you do it right.  It's not even particularly difficult;
it's just slightly beyond the scope of the course.  The initial 
motion of the four cars is very simple in the rotating frame: 
The cars are stationary.  People are naturally attracted to 
the simple description, as they should be.

There is also, of course, nothing wrong with using an inertial
frame.

The important rule is simple:  Do not mix-and-mismatch.  It 
is not OK to take a number from one reference frame and try 
to use it in another.  Non-spinning relative to the rotating
reference frame does not mean non-spinning relative to the
inertial reference frame.

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

A reminder to all:

 ++ It is OK to say that rotating frames are beyond the 
  scope of the course.
 ++ It is OK to say that centrifugal force is beyond the
  scope of the course.
 ++ It is OK to say that Coriolis effects are beyond the
  scope of the course.

The centrifugal field and the Coriolis effect do not depend
on the motion of any particular object;  they depend on the
motion of the /reference frame/ you choose to use.

In contrast:

 -- Pleeeease do not tell students that it is impossible
  to do physics in a rotating frame.
 -- Pleeeease do not tell students that centrifugal force
  does not exist.
 -- Pleeeease do not tell students that Coriolis effects
  do not exist.

You've got nothing to gain and much to lose by saying such
things.

First of all, with any luck, they won't believe you.  They have 
too much experience riding in cars and riding on playground 
merry-go-rounds.  They'll just think you're crazy.

If they are unfortunate enough to believe you, they'll just 
have to unlearn it all later.