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Rotational Motion question

2021 · 20 Jul · Shift 2 · Q67
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  5. /2021 · 20 Jul · Shift 2 · Q67

Rotational Motion question

2021 · 20 Jul · Shift 2 · Q67

JEE MainPhysicsRotational MotionNumerical+4 / −1
Two bodies, a ring and a solid cylinder of same material are rolling down without slipping an inclined plane. The radii of the bodies are same. The ratio of velocity of the centre of mass at the bottom of the inclined plane of the ring to that of the cylinder is x2{{\sqrt x } \over 2}2x​​. Then, the value of x is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 3

  1. Use energy conservation for rolling without slipping

For a body rolling down from height hhh without slipping:

mgh=12mv2+12Iω2mgh = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2mgh=21​mv2+21​Iω2

Since rolling without slipping,

ω=vR\omega = \frac{v}{R}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 ring

For a ring,

Iring=mR2I_{\text{ring}} = mR^2Iring​=mR2

Thus,

vr2=2mghm+m=2mgh2m=ghv_r^2 = \frac{2mgh}{m + m} = \frac{2mgh}{2m} = ghvr2​=m+m2mgh​=2m2mgh​=gh

So,

vr=ghv_r = \sqrt{gh}vr​=gh​


  1. For the solid cylinder

For a solid cylinder,

Icyl=12mR2I_{\text{cyl}} = \frac{1}{2}mR^2Icyl​=21​mR2

Thus,

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=4gh3=23ghv_c = \sqrt{\frac{4gh}{3}} = \frac{2}{\sqrt{3}}\sqrt{gh}vc​=34gh​​=3​2​gh​


  1. Take the ratio

vrvc=gh23gh=32\frac{v_r}{v_c} = \frac{\sqrt{gh}}{\frac{2}{\sqrt{3}}\sqrt{gh}} = \frac{\sqrt{3}}{2}vc​vr​​=3​2​gh​gh​​=23​​

Given,

vrvc=x2\frac{v_r}{v_c} = \frac{\sqrt{x}}{2}vc​vr​​=2x​​

Therefore,

x2=32\frac{\sqrt{x}}{2} = \frac{\sqrt{3}}{2}2x​​=23​​

So,

x=3⇒x=3\sqrt{x} = \sqrt{3} \Rightarrow x = 3x​=3​⇒x=3


  1. Final answer

3\boxed{3}3​

The derived answer matches the stored correct answer.

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