Re: Wave phase reversal on reflection

[John Barrer <forcejb@YAHOO.COM>]

If you "model" the medium as a series of particles
connected by springs, the explanation described by Bob
becomes even more lucid IMHO for intro students.

--- Bob Sciamanda <trebor@VELOCITY.NET> wrote:
> Yes, but what is the physical mechanism whereby the
> cancelling, reflected
> wave is generated?
> Perhaps something like:  When an upward going
> vertical pulse on a rope
> encounters a rigid wall termination, the last rope
> segment exerts an upward
> force on the wall.  The wall responds by exerting a
> downward pull on the
> rope segment (by N3), thus starting a downward going
> vertical pulse on the
> rope.
>
> The mathematical argument shows how WE KNOW that
> some such mechanism must be
> acting; only the physics (Newton's laws in the rope
> case) can identify that
> mechanism.
>
> The theorist is often completely satisfied with only
> the mathematical
> answer;  the unwashed student has yet not acquired
> sufficiently naive faith
> in mathematical models; he still requires the
> confirmation of a physical
> mechanism.
>
> Bob
>
> Bob Sciamanda (W3NLV)
> Physics, Edinboro Univ of PA (em)
> trebor@velocity.net
> www.velocity.net/~trebor
> ----- Original Message -----
> From: "Jack Uretsky" <jlu@HEP.ANL.GOV>
> To: <PHYS-L@lists.nau.edu>
> Sent: Tuesday, April 25, 2000 8:46 PM
> Subject: Re: Wave phase reversal on reflection
>
>
> > I thought that cancellation of the transmitted
> wave is about as physical
> > as one can get.
> >                                 Jack
> >
> > Adam was by constitution and proclivity a
> scientist; I was the same, and
> > we loved to call ourselves by that great
> name...Our first memorable
> > scientific discovery was the law that water and
> like fluids run downhill,
> > not up.
> >                           Mark Twain, <Extract
> from Eve's Autobiography>
> > On Tue, 25 Apr 2000, Bob Sciamanda wrote:
> >
> > > It is interesting that all of the responses to
> this query concern
> > > arguments based upon the need to satisfy certain
> mathematical
> requirements
> > > of  the wave function.    It is my experience
> that the student asking
> such
> > > questions is seeking a concrete PHYSICAL
> MECHANISM behind the behavior
> in
> > > question.  The answer to this aspect of the
> question will be different
> for
> > > each different kind of wave phenomenon ( eg:
> acoustical vs E/M ).
> > > This is a common pedagogical communication
> problem - we should try to
> get
> > > more concretely physical in addressing student
> queries (without
> prejudice
> > > to the guidance of mathematical models).  The
> student has not yet become
> > > sufficiently jaded into abstract, mathematical
> model thinking to be
> > > persuaded that they EXPLAIN the physical
> mechanisms of his concrete,
> > > conceptual questions.
> > >
> > > Bob
> > >
> > > Bob Sciamanda (W3NLV)
> > > Physics, Edinboro Univ of PA (em)
> > > trebor@velocity.net
> > > http://www.velocity.net/~trebor
> > >
> > > ----- Original Message -----
> > > From: "Jack Uretsky" <jlu@HEP.ANL.GOV>
> > > To: <PHYS-L@lists.nau.edu>
> > > Sent: Tuesday, April 25, 2000 2:34 PM
> > > Subject: Re: Wave phase reversal on reflection
> > >
> > >
> > > > Remove the boundary.  The reflected wave
> projected past the boundary
> > > > must exactly cancel the incident wave in the
> region beyond the
> boundary.
> > > >                         Regards,
> > > >                                 Jack
> > > >
> > > > Adam was by constitution and proclivity a
> scientist; I was the same,
> and
> > > > we loved to call ourselves by that great
> name...Our first memorable
> > > > scientific discovery was the law that water
> and like fluids run
> > > downhill,
> > > > not up.
> > > >                           Mark Twain, <Extract
> from Eve's
> Autobiography>
> > > > On Mon, 24 Apr 2000, SSHS KPHOX wrote:
> > > >
> > > > > My colleague asked if there were a good
> explanatiion for the phase
> > > > > reversal when a wave reflects from a
> boundary where the new speed
> will
> > > be
> > > > > less and not when going from slow to fast. I
> don't have one for her.
> > > Can
> > > > > anyone help?
> > > > >
> > > > > Ken Fox
> > > > > AP/IB Physics Teacher
> > > > > Smoky Hill High School, CO

__________________________________________________
Do You Yahoo!?
Send online invitations with Yahoo! Invites.
http://invites.yahoo.com