The Angle of Repose

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Granular materials like sand tend to form heaps when poured. The steepness of the heap at rest is described by the angle of repose, which is determined by a balance between gravity, normal force, and friction on the grains. When a heap of grains is disturbed, it can trigger an avalanche. As can be seen in the video above, avalanches are a surface phenomenon, only moving the top few layers of grain while most of the heap remains stationary.  (Video credit: Peddie School Physics)

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