[Phys-l] Paradoxes, or not. (Was: invariant mass...)

[John Denker <jsd@av8n.com>]

On 02/26/2008 09:55 AM, Rick Tarara wrote:

> Second--the paradoxes are the 'fun' part of this material--at least for the 
> students named above.

Rhetorical question:  Well, then, why not introduce other
subjects -- such as elementary mechanics -- in terms of 
paradoxes?  It's easy to do;  I can give you plenty of 
mechanical paradoxes if you want.  For example:

 On board an airliner, I throw a sticky wad of gum at the
 forward bulkhead.  It hits the bulkhead and sticks.  Its 
 kinetic energy (in the lab frame) goes from 700^2 m/2 
 to 600^2 m/2 for a delta of 130,000 m/2.  If you do the
 same calculation in the frame of the airliner, the delta
 is only 10,000 m/2.  This violates conservation of energy
 and/or Galileo's principle of relativity.

My father came up with that one.  He didn't know the answer.
It really bugged him.  He foisted it on me when I was about
10 years old, and it bugged me, too.  It took me several
years to figure it out.

To answer my own rhetorical question:  I think teaching
paradoxes (in an introductory course) is the epitome of bad
pedagogy.  It just sows confusion.  Where's the fun in that?   
Maybe if I were a sadist I would enjoy tormenting the students.  
Maybe if I were really, really insecure I would need a way 
to confuse the students so I would look smart by comparison.  
Either way it sounds like bad pedagogy to me.

IMHO students are confused enough already.

The correctly-stated laws of physics do not lead to paradoxes.
The incorrectly-stated laws of physics are full of paradoxes.
Why should we give the students practice misstating the laws
of physics?

One of the guys who taught me relativity said that his goal
was to get us to the point where we couldn't even state a
paradox ... since a correctly-stated description of any
situation is non-paradoxical.

   You can make a case that _between ourselves_ we should
   discuss paradoxes and misconceptions, but that's not
   the same as foisting paradoxes on intro-level students.  
   Not by a wide margin.