Due to the existing of nonlinear friction, it is difficult to model in the traditional way.

由于非线性摩擦力的存在，很难用传统的方法对其进行建模。

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On the nonlinear oscillation of internal sliding friction in particulate matter;

实验发现，棒在颗粒物质中运动时滑动摩擦力具有准周期振荡性质。

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