Question

Two thin discs each of mass 4.0 kg and radius 0.4 m are attached as shown in figure to form a rigid body. What is the rotational inertia of this body about an axis perpendicular to the plane of disc $B$ and passing through its center?

• A. $1.6kg-m^2$
• B. $3.2kg-m^2$
• C. $6.4kg-m^2$
• D. $0.8kg-m^2$

Answer 1

The correct option is B $3.2kg-m^2$

$I_B$ = $I$ of disc $B$ about centre of mass + $I$ of disc $A$ about centre of mass + mass of disc $A$ $\times$ (distance between centres of $A$ and $B$)${}^2$

= $\frac{m{R}^2}{2}+\frac{m{R}^2}{2}+m(2R{)}^2$ = $5m{R}^2$ = $5\times 4\times (0.4{)}^2$ = $3.2kg-m^2$