Physicists whose work requires thousands or millions of dollars of apparatus may be chagrined and/or delighted to learn of a new and startlingly counterintuitive effect discovered, by Raphaël Clément and co-workers at France’s Université Paris Diderot, using equipment that is quite literally sandbox-simple. Besides the doing-a-lot-with-a-little angle, their discovery makes for a potentially lucrative bar bet:
If you take two empty cans, one closed on the bottom and the other open (i.e. a tube), and turn them upside down, which will be harder to push into a bucket of sand? If, reasoning by analogy to liquids, you (like most people), said the closed one…well, you can sort of guess where this is heading: A closed can is, in fact, easier to push into a bed of sand than an open tube. Given the usual fine print. Adrian Cho explains over at ScienceNOW:
[T]he air trapped in the closed-ended cylinder affects the sand in a dramatic way, the researchers argue. Ordinarily, still sand acts somewhat like a solid. But if the air around the sand grains moves quickly enough, the sand will flow like a fluid, as physicists and engineers have long known. And when the closed-ended cylinder sinks into the sand, the air trapped in it rushes out from beneath the cylinder’s rim, fluidizing the sand at that point and making it easier for the stuff to get out of the can’s way. The sand has to be relatively densely packed, and the can must drop quickly. If the researchers slowly added weight to the can, the air seeped away gradually and the can descended no farther than the tube.
The paper is reportedly in press at Physical Review Letters, but seeing is believing. Try it yourself! Or, as a last resort, I suppose you could just watch the video.
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