New trends in physics teaching, v.4; The ... - unesdoc - Unesco

String and tape experiments
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marbles
tape
Figure 10.
Try producing standing waves. The end marble, which was stationary before, now moves more
than all the rest - so the mode which was at the end has now become an antinode.
If the device is hung vertically, and the lowest rubber band rapidly twisted by rolling it
between the thumb and forefinger, the bottom clip and marbles spin rapidly, and pass this
motion very slowly up the chain. At the top, the pulse reverses, and the marbles spin in the
opposite sense. Reaching the bottom, which hangs free, the rotational pulse reflects as with an
open end, the marbles continue to spin and wind up in the same sense, the pulse again travelling
up the chain, and reversing at the top. This proves a quite dramatic demonstration of the difference
in reflection between an open and closed end.
Waves in pipes
Having examined longitudinal waves, let us study their application in sound.
The vibration of air in tubes is the mechanism whereby virtually all wind instruments sound.
It also provides an excellent example of standing waves. All you need for these experiments are a
few drinking straws, a sheet of paper and a pair of scissors. We shall study tubes open at both
ends and those closed at one end; closed at both ends does not work because the sound cannot
get out! The frequency of the note produced by blowing gently across the end of a drinking
straw is that of the first normal mode of this tube. It takes a little practice to draw the note out
of this instrument; you may have to vary the angle at which you blow and the distance from your
lips in order to hear it, but with a little practice you should have no difficulty. The wavelength
X of the first standing wave in an open pipe is twice the length of the pipe. Now, close one end
of the straw while blowing across the other. The pitch falls an octave, since the first normal mode
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