Problem 1.106

A small disc $A$ is placed on an inclined plane forming an angle $\alpha$ with the horizontal (Fig. 1.27$)$ and is imparted an initial velocity $v_{0} .$ Find how the velocity of the disc depends on the angle $\varphi$ if the friction coefficient $k=\tan \alpha$ and at the initial moment $\varphi_{0}$ $=\pi / 2$