Re: Waves - and momentum + wave speed in the rod (Eric's note)

[Allen Brown <brownan@EDUC.QUEENSU.CA>]

Additionally, if you connect a wire to the head of the metal swinging
hammer, and another to the metal rod itself, and then use a battery and
oscilloscope with appropriate triggering so as to show a trace for the
contact time of the hammer on the rod, then that contact time corresponds
to the length of time it takes the sound wave to travel from the hammer
end to the free and and reflect back to the hammer end again.  This is
because the reflected wave, on arrival back at the hammer in contact with
the "source end" bounces the hammer away slightly.  The 'scope trace
corresponds to the to-and-fro time for wave travel.  It can be captured
with a storage scope, or the Vernier apparatus (the fast interface not the
old serial one), or even just be repeated trials and marking on the scope
face with a water soluble pen.  It's neat.

So, you can get the speed of sound in the rod from an indirect
time-of-flight type measurement.

Allen Brown

On Thu, 11 Mar 1999, Eric T. Lane wrote:

> One way to see the momentum carried by a wave is to use a long rod of
> some metal, say aluminum or steel.  Use a bit of vasoline to stick a
> small plug or weight on the end of the rod.  Then hit the other end of
> the rod with a hammer.  The momentum contained in the wave will be
> traped in the plug which will pop off the end of the rod with some
> velocity.  Expreiment with the mass of the plug to maximize the transfer
> of momentum, basically impedance matching.
>
> Prof. Eric T. Lane  318 Grote Hall  423-755-4523   eric-lane@utc.edu
> Physics Department, University of Tennessee at Chattanooga, TN 37403