why does something like this happen?
I have some physics knowledge, but I am not that advanced...
That's a very neat video. Here's what I think is happening:
It has nothing to do with the rice. Basically the frequency is creating a standing wave on the 2-d surface which has nodes in the shape given so the rice will bounce around erratically unless it settles on a point on the surface that isn't vibrating and then it'll just stay there. The one thing that confuses me is that the sound you hear in the video seems to be only one pitch and far too high. I'd think there's actually a lot more complicated signal going on in the low frequencies (below human hearing) to create this effect.
Reminds me of some cool "corn-starch" videos that I saw on the same site.
And if that doesn't convince you there's something strange about corn-starch (heh heh)
I agree with Starstrider. We did this with grains of sand on an aluminum plate in freshman physics.
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