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Re: standing waves (stiff media)

At 9:51 AM -0500 1/24/00, John Denker wrote:

The directly-contrasting demonstration is more difficult and less pleasing,
because it tries to demonstrate the NONexistence of such a solution -- and
it's always hard to prove a negative. Get a long metal or fiberglass rod
and clamp it at one end so it sticks across the classroom. It needs to be
stiff enough to keep itself off the floor, but floppy enough to allow
flexional waves. If you smack it with a bat, you will *not* see a
shape-preserving wave packet run to the other end. The wavespeed for the
high-frequency components is so much faster than the wavespeed for the
low-frequency components that the wave packet tears itself apart.


and it IS interesting to set up a STANDING wave in one of these STIFF media.

We have several really stiff, tight, heavy springs from lousy,
expensive windows that were installed in our school. These 'sash'
springs seem to break on a regular basis and I have collected some
broken ones.

We can carefully bend one 'double' and latch the broken tip into the
unbroken attachment loop on the other end. Then when it is dropped
onto the floor, it 'springs' back straight with great force and sets
up a standing wave that lasts for a few seconds before damping.

Clearly showing the translational nodes (~22% in) and the
translational antinodes at the middle and ends.

These springs probably work better than the fiberglass or metal rod
since they start with zero translational momentum and don't need an
external impulse to excite them.

But the long rods MIGHT be interesting if hung ~22% of the way in
from each end and given a whack at the center.

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britton@academic.ncssm.edu you have forgotten everything
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(919) 286-3366 x224 Albert Einstein, 1936