Question

A circular disc $A$ and a ring $B$ have same mass and same radius. If they are rotated with the same angular speed about their own axis, then

• A. $A$ has less moment of inertia than $B$
• B. $A$ has less rotational kinetic energy than $B$
• C. $A$ and $B$ have the same angular momentum
• D. $A$ has greater angular momentum than $B$
• E. $A$ has the same moment of inertia as that of $B$

Answer 1

Answer: (A), (B)

The correct options are
A $A$ has less rotational kinetic energy than $B$
B $A$ has less moment of inertia than $B$

Moment of inertia of disc
$I_{disc}=\frac{M{R}^2}{2}$
Moment of inertia of ring $I_{ring}=M{R}^2$
Therefore, $A$ has less moment of inertia than $B$ because of which $A$ has less rotational kinetic energy than $B$.