Question

A round disc of moment of inertia $I_2$ about its axis perpendicular to its plane and passing through its centre is placed over another disc of moment of inertia $I_1$ rotating with an angular velocity $\omega$ about the same axis. The final angular velocity of the combination of discs is

• A. $\frac{I_2\omega}{I_1+I_2}$
• B. $\omega$
• C. $\frac{I_1\omega}{I_1+I_2}$
• D. $\frac{(I_1+I_2)\omega}{I_1}$

Answer 1

Answer: (C)

$\frac{I_1\omega}{I_1+I_2}$

Answer 2

Applying conservation of angular momentum.
$I_1\omega=(I_1+I_2)\omega_1$
$\omega_1=\frac{I_1\omega}{(I_1+I_2)}$.