A thin uniform circular disc of mass ' $\mathrm{M}$ ' and radius ' $R$ ' is rotating with angular velocity '…

A thin uniform circular disc of mass ' $\mathrm{M}$ ' and radius ' $R$ ' is rotating with angular velocity ' $\omega$ ', in a horizontal plane about an axis passing through its centre and perpendicular to its plane. Another disc of same radius but of mass $\left(\frac{M}{2}\right)$ is placed gently on the first disc co-axially. The new angular velocity will be
  1. $\frac{2}{3} \omega$
  2. $\frac{4}{5} \omega$
  3. $\frac{5}{4} \omega$
  4. $\frac{3}{2} \omega$

Solution

Angular momentum $=\mathrm{I} \omega$ By conservation of angular momentum, $\mathrm{I}_1 \omega_1=\mathrm{I}_2 \omega_2$ Here, $I_1=\frac{M^2}{2}, I_2=\frac{(M+M / 2)}{2} R^2=\frac{3 M^2}{4}$ $\begin{aligned} & \therefore \quad \frac{\mathrm{MR}^2}{2} \omega_1=\frac{3 \mathrm{MR}^2}{4} \omega_2 \\ & \therefore \quad \omega_2=\frac{2}{3} \omega_1 \end{aligned}$

Asked in: MHT CET 2023 (12 May Shift 1)

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