An unfixed spring of stiffness k and mass M lies on a smooth horizontal table. A thin, inextensible thread is tied to one of its ends and thrown over a fixed block mounted on the edge of the table. The thread hangs vertically from it. A weight of mass m is attached to the hanging end of the thread, which at a certain moment is released without an initial speed. Determine the elongation of the spring during movement. Its rigidity should be considered sufficient so that the elongation is small compared to the original length.

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