A thin uniform square lamina of side $a$ is placed in the $x y$-plane with its sides parallel to $X$ and…

A thin uniform square lamina of side $a$ is placed in the $x y$-plane with its sides parallel to $X$ and $Y$-axes and with its centre coinciding with origin. Its moment of inertia about an axis passing through a point on the $Y$-axis at a distance $Y=2 a$ and parallel to $X$-axis is equal to its moment of inertia about an axis passing through a point on the $X$-axis at a distance $x=d$ and perpendicular to $x y$-plane. Then value of $d$ is
  1. $\frac{7}{3} a$
  2. $\frac{\sqrt{47}}{12} a$
  3. $\frac{9}{5} a$
  4. $\frac{\sqrt{51}}{12} a$

Solution

$\begin{array}{rlrl}\frac{m a^2}{12}+m\left(4 a^2\right) & =\frac{m a^2}{6}+m d^2 \\ & \text { or } \quad d^2 & =\frac{47 a^2}{12} \\ \therefore & d & =\sqrt{\frac{47}{12}} a\end{array}$

Asked in: AP EAMCET 2004

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