When I rub my finger against a piece of rubber, and it makes a tone of 2 kHz, what is vibrating at 2,000 times per second in order to produce that tone? The rubber doesn't feel flexible enough to move with my finger easily, especially at such high frequencies.
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asked Nov 3, 2014 at 2:01
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1 Answer
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What you are probably observing is a stick-slip phenomenon (sometimes called a relaxation oscillator - think chalk on blackboard).
The whole point is that high modulus ("not flexible enough" == high modulus) is just what you need to get such high frequencies: a small displacement must give rise to a large force.
I must say I'm surprised you can do this by rubbing your finger - because no matter how stiff the rubber is, most fingers are not that stiff... and it's the composite modulus that matters.
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1$\begingroup$ You can definitely do it with your finger, for example you can make beautiful noises rubbing it on a glass, but it is not so easy. $\endgroup$– agemOCommented Nov 3, 2014 at 9:29
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$\begingroup$ maybe adding some extra material in between (e.g glue or sth like that can make it even easier) $\endgroup$– Nikos M.Commented Nov 3, 2014 at 13:13
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$\begingroup$ That's a little above my head. Just to get things straight, the material is actually moving at that high frequency, right? $\endgroup$ Commented Nov 3, 2014 at 14:28
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$\begingroup$ Yes that's right. It's the same mechanism that makes the bow of a violin work - it sticks, it deflects, it jumps a bit, then sticks again. Repeat. It's why you need resin on the violin bow (and why Nikos recommended "something sticky" in his comment). $\endgroup$– FlorisCommented Nov 3, 2014 at 14:30