Problem 1.69

A bar of mass \(m\) resting on a smooth horizontal plane starts moving due to the force \(F=m g / 3\) of constant magnitude. In the process of its rectilinear motion the angle \(\alpha\) between the direction of this force and the horizontal varies as \(\alpha=a s,\) where \(a\) is a constant, and \(s\) is the distance traversed by the bar from its initial position. Find the velocity of the bar as a function of the angle \(\alpha\).