Q23. A circular disc reaches from top to bottom of an inclined plane of length $l$. When it slips down the plane, if takes $t \mathrm {~s}$. When it rolls down the plane then it takes $\left( \frac { \alpha } { 2 } \right) ^ { 1 / 2 } t \mathrm {~s}$, where $\alpha$ is $\_\_\_\_$\\