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Rotational Motion question

2022 · 28 Jun · Shift 1 · Q46
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  5. /2022 · 28 Jun · Shift 1 · Q46

Rotational Motion question

2022 · 28 Jun · Shift 1 · Q46

JEE MainPhysicsRotational MotionMCQ+4 / −1

Match List-I with List-II

List-I List-II
(A) Moment of inertia of solid sphere of radius R about any tangent. (I) 53MR2{5 \over 3}M{R^2}35​MR2
(B) Moment of inertia of hollow sphere of radius (R) about any tangent. (II) 75MR2{7 \over 5}M{R^2}57​MR2
(C) Moment of inertia of circular ring of radius (R) about its diameter. (III) 14MR2{1 \over 4}M{R^2}41​MR2
(D) Moment of inertia of circular disc of radius (R) about any diameter. (IV) 12MR2{1 \over 2}M{R^2}21​MR2

Choose the correct answer from the options given below :

  1. A
    A - II, B - I, C - IV, D - III
  2. B
    A - I, B - II, C - IV, D - III
  3. C
    A - II, B - I, C - III, D - IV
  4. D
    A - I, B - II, C - III, D - IV
View written solutionFree

Correct answer: A

  1. Use standard moments of inertia and parallel axis theorem

We match each item in List-I.


  1. (A) Solid sphere of radius RRR about any tangent

For a solid sphere, moment of inertia about any diameter is: Idiameter=25MR2I_{\text{diameter}}=\frac{2}{5}MR^2Idiameter​=52​MR2

A tangent axis is parallel to a diameter and at distance RRR from the center, so by parallel axis theorem: Itangent=Idiameter+MR2I_{\text{tangent}}=I_{\text{diameter}}+MR^2Itangent​=Idiameter​+MR2 Itangent=25MR2+MR2=75MR2I_{\text{tangent}}=\frac{2}{5}MR^2+MR^2=\frac{7}{5}MR^2Itangent​=52​MR2+MR2=57​MR2

So, A→(II)A \to (II)A→(II)


  1. (B) Hollow sphere of radius RRR about any tangent

For a hollow sphere (spherical shell), moment of inertia about any diameter is: Idiameter=23MR2I_{\text{diameter}}=\frac{2}{3}MR^2Idiameter​=32​MR2

Again using parallel axis theorem for tangent axis: Itangent=23MR2+MR2=53MR2I_{\text{tangent}}=\frac{2}{3}MR^2+MR^2=\frac{5}{3}MR^2Itangent​=32​MR2+MR2=35​MR2

So, B→(I)B \to (I)B→(I)


  1. (C) Circular ring of radius RRR about its diameter

For a ring, moment of inertia about axis perpendicular to its plane through center is: Iz=MR2I_z=MR^2Iz​=MR2

By perpendicular axis theorem for a planar body: Iz=Ix+IyI_z=I_x+I_yIz​=Ix​+Iy​

Since for a ring, both diameters are equivalent, Ix=IyI_x=I_yIx​=Iy​

Hence, MR2=2IdiameterMR^2=2I_{\text{diameter}}MR2=2Idiameter​ Idiameter=12MR2I_{\text{diameter}}=\frac{1}{2}MR^2Idiameter​=21​MR2

So, C→(IV)C \to (IV)C→(IV)


  1. (D) Circular disc of radius RRR about any diameter

For a disc, moment of inertia about axis perpendicular to plane through center is: Iz=12MR2I_z=\frac{1}{2}MR^2Iz​=21​MR2

Using perpendicular axis theorem, Iz=Ix+IyI_z=I_x+I_yIz​=Ix​+Iy​

By symmetry, Ix=Iy=IdiameterI_x=I_y=I_{\text{diameter}}Ix​=Iy​=Idiameter​

So, 12MR2=2Idiameter\frac{1}{2}MR^2=2I_{\text{diameter}}21​MR2=2Idiameter​ Idiameter=14MR2I_{\text{diameter}}=\frac{1}{4}MR^2Idiameter​=41​MR2

Thus, D→(III)D \to (III)D→(III)


  1. Final matching

A→II,B→I,C→IV,D→IIIA \to II, \quad B \to I, \quad C \to IV, \quad D \to IIIA→II,B→I,C→IV,D→III

This corresponds to Option A.


  1. Comparison with stored answer

Stored correct answer = A.

Our derived answer = A.

So the answer agrees with the stored answer.

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