Since the cylinder moves without sliding, the centre of the cylinder rotates about the point $O$, while passing through the common edge of the planes. In other words, the point $O$ becomes the foot of the instantaneous axis of rotation of the cylinder. It at any instant during this motion the velocity of the C.M. is $v_{1}$ when the angle (shown in the figure) is $\beta$, we have
$$
\frac{m v_{1}^{2}}{R}=m g \cos \beta-N
$$