Re: Wave phase reversal on reflection

[Bob Sciamanda <trebor@VELOCITY.NET>]

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