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A system is in equilibrium when the forces constituting it are arranged in such a way as to compensate each other, like the two weights pulling at the arms of a pair of scales.

当组成一个系统的力以一种互相补偿的方式排列时，这个系统就处于平衡状态，就像两个砝码拉着一对天平的扶手一样。

— Rudolf Arnheim 鲁道夫·阿恩海姆