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JEE Main20206 Sep 2020Evening ShiftPhysicsRotational MotionActual

The linear mass density of a thin rod AB of length L varies from A to B as λ x = λ 0 1 + x L , where x is the distance from A . If M is the mass of the rod then its moment of inertia about an axis passing through A and perpendicular to the rod :

Options

  1. A5 12 ML 2
  2. B7 18 ML 2
  3. C2 5 ML 2
  4. D3 7 ML 2

Correct answer

B. 7 18 ML 2

Step-by-step solution

dm = λ . dv dI = dm . x 2 = λ 0 1 + x L . x 2 dx I = λ 0 ∫ 0 1 x 2 + x 3 L dx = λ 0 x 3 3 + x 4 4 L 0 L = λ 0 L 3 3 + L 3 4 I = 7 λ 0 L 3 12 m = ∫ 0 L λdx = 3 λ 0 L 2 I m = 7 L 2 12 × 3 2 = 7 L 2 18 ⇒ I = 7 18 mL 2

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