The frictional force remains constant, dependent on the friction, the kinetic friction coefficient, is always the same...

摩擦力不守恒，它取决于摩擦力，动能摩擦系数，一直是相同的。

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It is pointed out that translational kinetic energy and rotational kinetic energy mutually transferred by sliding friction force, but it only has positive acting.

讨论滑动摩擦力作功，说明了滑动摩擦力可以使刚体的平动动能和转动动能之间相互转换，但总是作负功。

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