A thin horizontal uniform rod AB of mass m and length l can rotate freely about a vertical axis

A thin horizontal uniform rod AB of mass m and length l can rotate freely about a vertical axis passing through its end A. At a certain moment the end B starts experiencing a constant force F which is always perpendicular to the original position of the stationary rod and directed in a horizonatal plane. Find the angular velocity of the rod as a function of its rotation angle `varphi` counted relative to the initial position.