A circular disc reaches from top to bottom of an inclined plane of length l. When it slips down the plane, if takes t s. When it rolls down the plane…

A circular disc reaches from top to bottom of an inclined plane of length $\displaystyle l$. When it slips down the plane, if takes $\displaystyle t \mathrm{~s}$. When it rolls down the plane then it takes $\displaystyle \left(\frac{\alpha}{2}\right)^{1 / 2} t \mathrm{~s}$, where $\displaystyle \alpha$ is $\displaystyle \_\_\_\_$.