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JEE Main 2026 (23 Jan, Shift 2) · Moment of Inertia & Radius of Gyration

Moment of Inertia & Radius of Gyration question from JEE Main 2026 (23 Jan, Shift 2)

Q39JEE Main 2026 · 23 JAN · SHIFT 2Difficult

Moment of Inertia & Radius of Gyration

Suppose there is a uniform circular disc of mass M kgM\text{ kg} and radius r mr\text{ m} shown in figure. The shaded regions are cut out from the disc. The moment of inertia of the remainder about the axis AA of the disc is given by x256Mr2\frac{x}{256}Mr^2. The value of xx is ______.

Figure

Numerical answer type. Work it out, then open the answer.

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Answer: 109

  1. Each hole: radius r4\frac{r}{4}, mass M16\frac{M}{16}, centre 3r4\frac{3r}{4} from axis A.
  2. One hole: 12M16r216+M169r216=19Mr2512\frac{1}{2}\frac{M}{16}\frac{r^2}{16} + \frac{M}{16}\frac{9r^2}{16} = \frac{19Mr^2}{512}
  3. Two holes: 19Mr2256\frac{19Mr^2}{256}
  4. I=128Mr2256−19Mr2256=109256Mr2I = \frac{128Mr^2}{256} - \frac{19Mr^2}{256} = \frac{109}{256}Mr^2, so x=109x = 109.
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