To a bar of mass m lying on a smooth horizontal plane, a constant modulo force F = mg / 3 was applied. In the process of its rectilinear motion, the angle a between the direction of this force and the horizon is changed according to the law a = ks, where k is the constant, s is the path traversed by the bar (from the initial position). Find the speed of the bar as a function of the angle a.

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