Two thin discs each of mass `M` and radius `r` are attached as shown in figure, to from a rigid body. The rotational inertia of this body about an axis perpendicular to the plane of disc `B` and passing through its centre is :
.

Options

A. `2 Mr^2`
B. `3 Mr^2`
C. `4 Mr^2`
D. `5 Mr^2`

Answer

D

Explanation

(d) `I = I_B + I_A = (1)/(2) Mr^2 + ((1)/(2) Mr^2 + M(2 r)^2) = 5 Mr^2`.