since \(\vec{\tau}=0, \therefore L=\) constant
\(\therefore I_{1} \omega_{1}=I_{2} \omega_{2}\)
Here \(I_{1}=\frac{1}{2} M R^{2}\)
\(\omega_{1}=\omega\)
\(I_{2}=\frac{1}{2} M R^{2}+\frac{1}{2} \frac{M}{4} R^{2}=\left(\frac{4+1}{8}\right) M R^{2}=\frac{5}{8} M R^{2}\)
\(\therefore \omega_{2}=\frac{I_{1} \omega_{1}}{I_{2}}=\frac{8}{2 \times 5} \omega=\frac{4}{5} \omega\)