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Rotational Motion question

2022 · 25 Jul · Shift 1 · Q45
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  5. /2022 · 25 Jul · Shift 1 · Q45

Rotational Motion question

2022 · 25 Jul · Shift 1 · Q45

JEE MainPhysicsRotational MotionMCQ+4 / −1
A solid cylinder and a solid sphere, having same mass MMM and radius RRR, roll down the same inclined plane from top without slipping. They start from rest. The ratio of velocity of the solid cylinder to that of the solid sphere, with which they reach the ground, will be :
  1. A
    53\sqrt{\frac{5}{3}}35​​
  2. B
    45\sqrt{\frac{4}{5}}54​​
  3. C
    35\sqrt{\frac{3}{5}}53​​
  4. D
    1415\sqrt{\frac{14}{15}}1514​​
View written solutionFree

Correct answer: D

  1. Use conservation of mechanical energy

For a body rolling without slipping from height hhh:

Mgh=12Mv2+12Iω2Mgh = \frac{1}{2}Mv^2 + \frac{1}{2}I\omega^2Mgh=21​Mv2+21​Iω2

Since rolling without slipping,

v=ωR⇒ω=vRv = \omega R \quad \Rightarrow \quad \omega = \frac{v}{R}v=ωR⇒ω=Rv​

So,

Mgh=12Mv2+12Iv2R2Mgh = \frac{1}{2}Mv^2 + \frac{1}{2}I\frac{v^2}{R^2}Mgh=21​Mv2+21​IR2v2​

Mgh=12v2(M+IR2)Mgh = \frac{1}{2}v^2\left(M + \frac{I}{R^2}\right)Mgh=21​v2(M+R2I​)

Hence,

v2=2MghM+I/R2v^2 = \frac{2Mgh}{M + I/R^2}v2=M+I/R22Mgh​


  1. For the solid cylinder

Moment of inertia about its axis:

Ic=12MR2I_c = \frac{1}{2}MR^2Ic​=21​MR2

Therefore,

vc2=2MghM+12M=2Mgh32M=4gh3v_c^2 = \frac{2Mgh}{M + \frac{1}{2}M} = \frac{2Mgh}{\frac{3}{2}M} = \frac{4gh}{3}vc2​=M+21​M2Mgh​=23​M2Mgh​=34gh​

So,

vc=4gh3v_c = \sqrt{\frac{4gh}{3}}vc​=34gh​​


  1. For the solid sphere

Moment of inertia about its axis:

Is=25MR2I_s = \frac{2}{5}MR^2Is​=52​MR2

Therefore,

vs2=2MghM+25M=2Mgh75M=10gh7v_s^2 = \frac{2Mgh}{M + \frac{2}{5}M} = \frac{2Mgh}{\frac{7}{5}M} = \frac{10gh}{7}vs2​=M+52​M2Mgh​=57​M2Mgh​=710gh​

So,

vs=10gh7v_s = \sqrt{\frac{10gh}{7}}vs​=710gh​​


  1. Find the ratio

vcvs=4gh310gh7\frac{v_c}{v_s} = \sqrt{\frac{\frac{4gh}{3}}{\frac{10gh}{7}}}vs​vc​​=710gh​34gh​​​

vcvs=43⋅710=2830=1415\frac{v_c}{v_s} = \sqrt{\frac{4}{3}\cdot\frac{7}{10}} = \sqrt{\frac{28}{30}} = \sqrt{\frac{14}{15}}vs​vc​​=34​⋅107​​=3028​​=1514​​


  1. Match with options

vcvs=1415\boxed{\frac{v_c}{v_s} = \sqrt{\frac{14}{15}}}vs​vc​​=1514​​​

So the correct option is D.

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