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Rotational Motion question

2021 · 24 Feb · Shift 1 · Q62
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  5. /2021 · 24 Feb · Shift 1 · Q62

Rotational Motion question

2021 · 24 Feb · Shift 1 · Q62

JEE MainPhysicsRotational MotionMCQ+4 / −1
Moment of inertia (M. I.) of four bodies, having same mass and radius, are reported as; I1 = M.I. of thin circular ring about its diameter, I2 = M.I. of circular disc about an axis perpendicular to disc and going through the centre, I3 = M.I. of solid cylinder about its axis and I4 = M.I. of solid sphere about its diameter. Then :
  1. A
    I1 = I2 = I3 > I4
  2. B
    I1 + I3 < I2 + I4
  3. C
    I1 = I2 = I3 < I4
  4. D
    I1 + I2 = I3 + 52{5 \over 2}25​ I4
View written solutionFree

Correct answer: A

  1. Write the standard moments of inertia for bodies of the same mass MMM and radius RRR.
  • Thin circular ring about a diameter: The M.I. of a ring about an axis through the center and perpendicular to its plane is MR2MR^2MR2. By perpendicular axis theorem, the M.I. about any diameter is I1=12MR2I_1 = \frac{1}{2}MR^2I1​=21​MR2

  • Circular disc about an axis perpendicular to the disc through the center: I2=12MR2I_2 = \frac{1}{2}MR^2I2​=21​MR2

  • Solid cylinder about its own axis: I3=12MR2I_3 = \frac{1}{2}MR^2I3​=21​MR2

  • Solid sphere about its diameter: I4=25MR2I_4 = \frac{2}{5}MR^2I4​=52​MR2

  1. Compare the values:

We have I1=I2=I3=12MR2I_1 = I_2 = I_3 = \frac{1}{2}MR^2I1​=I2​=I3​=21​MR2 and I4=25MR2I_4 = \frac{2}{5}MR^2I4​=52​MR2

Since 12>25\frac{1}{2} > \frac{2}{5}21​>52​ we get I1=I2=I3>I4I_1 = I_2 = I_3 > I_4I1​=I2​=I3​>I4​

So, Option A is correct.

  1. Check the other options:
  • Option B: I1+I3=12MR2+12MR2=MR2I_1 + I_3 = \frac{1}{2}MR^2 + \frac{1}{2}MR^2 = MR^2I1​+I3​=21​MR2+21​MR2=MR2 I2+I4=12MR2+25MR2=910MR2I_2 + I_4 = \frac{1}{2}MR^2 + \frac{2}{5}MR^2 = \frac{9}{10}MR^2I2​+I4​=21​MR2+52​MR2=109​MR2 So, I1+I3<I2+I4I_1 + I_3 < I_2 + I_4I1​+I3​<I2​+I4​ becomes MR2<910MR2MR^2 < \frac{9}{10}MR^2MR2<109​MR2 which is false.

  • Option C: It says I1=I2=I3<I4I_1 = I_2 = I_3 < I_4I1​=I2​=I3​<I4​, but actually 12MR2>25MR2\frac{1}{2}MR^2 > \frac{2}{5}MR^221​MR2>52​MR2. False.

  • Option D: I1+I2=12MR2+12MR2=MR2I_1 + I_2 = \frac{1}{2}MR^2 + \frac{1}{2}MR^2 = MR^2I1​+I2​=21​MR2+21​MR2=MR2 I3+52I4=12MR2+52⋅25MR2=12MR2+MR2=32MR2I_3 + \frac{5}{2}I_4 = \frac{1}{2}MR^2 + \frac{5}{2}\cdot \frac{2}{5}MR^2 = \frac{1}{2}MR^2 + MR^2 = \frac{3}{2}MR^2I3​+25​I4​=21​MR2+25​⋅52​MR2=21​MR2+MR2=23​MR2 Not equal. False.

  1. Final answer:

The correct option is A\boxed{A}A​

  1. Comparison with stored answer:

Stored correct answer: A

My derived answer: A

They agree.

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