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Breaking glass with sound...
Rubbing the rim of a wine glass with a wet finger will cause it to resonate at its resonant frequency. The glass is placed in front of a speaker playing a sine wave, created by the function generator, of this same frequency. When the amplitude is turned up, we can see by shining a strobe light at the glass that this resonant frequency causes it to oscillate. When the glass becomes too stressed, it will shatter, which we see very clearly on high speed video.A few things to note: The scrolling effect seen in the strobe light footage is caused by interference between the strobe light frequency and the video camera frame rate. Also, the real oscillations of the glass are much faster than they appear in the strobe footage. Setting the frequency of the strobe light can make them appear much slower so that we can see the oscillations in real time without the help of high speed video.
I wonder how many PC screens just blew?
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Behold, power of video blogs! What an opportunity to learn these days. Twenty years ago I could only dream about such an experiment in school, and those educational movies during physics class were rare and bleak. (Comedy movies with opera singers breaking glasses don't count.) Neither did the situation change ten years ago at a university. My own kids will definitely see more. I envy them :)
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1.) Both glasses broke starting from the bottom where the stem attached and propagated up to the rim. Greatest displacement was on the rim, where you'd think the greatest stress would be.
2.) The major broken pieces continued to oscillate well after structural failure, giving each piece a pulsing life of its own.
Now we can go into in depth study to see if it's cross section, or left over stresses from annealing that cause this progression of stress failure.
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