Re: [Phys-L] the best way to debunk myths

[brian whatcott <betwys1@sbcglobal.net>]

On 12/5/2014 12:21 PM, John Denker wrote in part:
> ///
> It is 100% OK for professionals to discuss misconceptions in
> a forum such as this, but there are tremendous inherent risks
> in mentioning misconceptions in front of naïve students. ///
>
> This makes contact with the discussion of "half" of a vortex
> ring.  The fact is, there is no such thing as half a vortex
> ring.  A vortex line cannot come to an end, not within the
> material or even at the boundary.  The fact that you cannot
> /see/ the continuation of the vortex line does not change the
> laws of physics.
>
> If you show the "half vortex" video to students, they will form
> a mental model of vortex lines ending abruptly at the boundary,
> and there will be hell to pay getting them to unlearn it.
>     "But I saw it with my own eyes."
> I say whatever you saw is what it is, but the description in
> terms of a half-vortex is not correct.   ////
This looks like an ideal opportunity to test the opposing constructions
given  to explain a mixed media vortex (half) ring.

Taking a card screen affixed to a light pole, one could launch a 
(half)-vortex
as before, and interpose the card above the water surface to see that the
 (half)-vortex is immediately quenched.

Then, supposing that the traveling (half)-vortex had not got the memo
about the continuity requirement, and so continued across the water,
 one might repeat the test, but this time insert the blocking card about
 one third the water depth as the distance between the vortices at the 
surface
 in order to counter the Sheldon-style explanation that a shallow
 near surface (half) vortex could be completing the continuity requirement.
Does anyone know how to contact the physics professional who arranged
 the initial video clip?    The opportunity to continue an investigation in
 this way seems so  - scientific -  in spirit somehow!    :-)

Brian Whatcott    Altus OK