Re: [Phys-l] mechanical (Applies to all, I presume.) on reflection question.

[brian whatcott <betwys1@sbcglobal.net>]

BC, you show a movie of an 'al fresco' version of the Shive machine, 
which was designed to provide a visual illustration of slow transverse 
wave propagation and reflections. I am afraid your note does not specify 
if the movie shows the effect of a fixed, or of a free end.  The 
illustration I provide (below) has three still pictures showing the 
effect of a fixed or a free end. [If your browser does not depict these 
three still pictures across the width of your display, you may find the 
description confusing.]

The fixed end reflects an inverted pulse (not 'phase-reversed' as 
described) whereas the free end reflects an isopolar pulse.
 An end provided with the same acoustic impedance as the loaded 
transmission rod would be expected to minimize reflections of either type.

<http://www.physics.umd.edu/lecdem/services/demos/demosg3/g3-03.htm>


Brian W

On 1/30/2011 4:53 PM, Bernard Cleyet wrote:
> A Phys. Teacher SOS thread is on a "cheap" * wave machine (the Bell Labs. type) made of skewers, gummy bears, and masking tape.**  One poster wrote,
>
> "He also didn't focus on the inversion that took place when the pulse reached the end. Suggest that you could produce more defined and shorter pulses that would show this phenomenon pretty clearly."
>
> Which I think clearly wrong***.  On reflection the up end of the skewers is still up.  This confirms the theory regarding free and clamped ends of the machine.  There does appear to be some loss in amplitude.  I haven't carefully looked to see if it's expectedly, at the end.  It is generally a rather low Q resonator.
>
>
> So my question is must a appropriately partially free (passive) end be completely absorbing.  i.e. one which results in neither inversion or non-inversion. /snip/
> *** View:
> http://www.cleyet.org/Someone_is_Wrong/Wave_Machine.mov