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Consider a thin uniform square sheet made of a rigid material. If its side is a, its mass m and its moment of inertia about one of its diagonals is I, then

Asked in JEE Main Online 2015 · Standard bodies and axis theorems

Answer: (3) I=(ma²)/(12)

Step-by-step solution

About the axis perpendicular to the sheet through its centre, a square plate of side a has I_z=m/(12)(a²+a²)=(ma²)/6.

The two diagonals are in-plane axes through the centre, perpendicular to each other.

Perpendicular-axis theorem: I_d₁+I_d₂=I_z.

The square is symmetric under a quarter turn, so the two diagonals give the same value: I_d₁=I_d₂=I.

2I=(ma²)/6, so I=(ma²)/(12).

This is the same value as about an axis through the centre parallel to a side — for a square, every in-plane axis through the centre gives the same moment of inertia.

Why the other options are wrong

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