The moment of inertia of a thin uniform rod of mass ' $M$ ' and length ' $L$ ', about an axis perpendicular…

The moment of inertia of a thin uniform rod of mass ' $M$ ' and length ' $L$ ', about an axis perpendicular to length of the rod and at a distance ' $\mathrm{L} / 4$ ' from one end is
  1. $\frac{\mathrm{ML}^2}{6}$
  2. $\frac{\mathrm{ML}^2}{12}$
  3. $\frac{7 \mathrm{ML}^2}{24}$
  4. $\frac{7 \mathrm{ML}^2}{48}$

Solution

Moment of inertia via parallel axis theorem:

The moment of inertia about the center of mass axis is $I_{\text{CM}} = \frac{ML^2}{12}$.

The distance from the given axis to the center of mass is $d = |\frac{L}{2} - \frac{L}{4}| = \frac{L}{4}$.

Applying the parallel axis theorem $I = I_{\text{CM}} + Md^2$:

$I = \frac{ML^2}{12} + M\left(\frac{L}{4}\right)^2 = \frac{ML^2}{12} + \frac{ML^2}{16}$

Combining terms with common denominator 48:

$I = \frac{4ML^2}{48} + \frac{3ML^2}{48} = \frac{7ML^2}{48}$

Final moment of inertia: $\frac{7ML^2}{48}$

Asked in: MHT CET 2025 (05 May Shift 2)

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