tailieunhanh - The Self-Made Tapestry Phần 8

Các nhà nghiên cứu đặt ra để xây dựng một mô hình lý thuyết đơn giản của quá trình này. Để làm như vậy, họ đã thiết lập các tiêu chí khi tuyết lở bắt đầu và dừng lại. Các tiêu chí này được khám phá đống hạt. | The researchers set out to construct a simple theoretical model of this process. To do so they had to establish criteria for when avalanches started and stopped. These criteria are well explored for piles of grains and you can see them for yourself by tipping up bowls of granulated sugar and long-grain rice until they undergo avalanches. First smooth the surfaces of the materials so that they are both horizontal. Then slowly tilt the bowls until a layer of grains shears off and runs out in an avalanche. There is a critical angle called the angle of maximum stability 0m at which sliding takes place. Moreover when the avalanche is over the slope of the grains in the bowl will have decreased to a value for which it is stable. This is called the angle of repose 0r and the slope always relaxes to this same angle Fig. . Both of these avalanche angles depend on the grain shapeyou ll find that 0m for rice is larger than that for sugar whereas granulated sugar caster sugar and couscous all with roughly spherical grains all have a similar angle within the accuracy of this kitchen-table demonstration. You can see the same thing by letting a steady trickle of sugar pass through a hole in a bag so that it forms a heap on the table top. The heap grows steeper and steeper until eventually there is a miniature landslide. Thereafter you ll find that however much more sugar you add the slope of the pile stays more or less constant as it grows with little landslides making sure that this is so. The angle of the steady slope is the angle of repose. Fig. The stratification that takes place when mixed grains are poured can be mimicked in a simple theoretical model in which the two grains have different shapes square and rectangular. The model assumes that as the are poured the grains stack up into colums with all of the rectangular grains upright a . Although this is a highly artificial assumption it reporduces the effect of different grain shapes which is the cause of the .

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