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Rotational Motion question

2024 · 4 Apr · Shift 1 · Q81
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  5. /2024 · 4 Apr · Shift 1 · Q81

Rotational Motion question

2024 · 4 Apr · Shift 1 · Q81

JEE MainPhysicsRotational MotionNumerical+4 / −1
A solid sphere and a hollow cylinder roll up without slipping on same inclined plane with same initial speed vvv. The sphere and the cylinder reaches upto maximum heights h1h_1h1​ and h2h_2h2​ respectively, above the initial level. The ratio h1:h2h_1: h_2h1​:h2​ is n10\frac{n}{10}10n​. The value of nnn is ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 7

  1. Use conservation of mechanical energy

Since both bodies roll without slipping, their initial total kinetic energy converts completely into gravitational potential energy at the highest point.

So, mgh=Ktrans+Krotmgh = K_{\text{trans}} + K_{\text{rot}}mgh=Ktrans​+Krot​

with Ktrans=12mv2,Krot=12Iω2K_{\text{trans}} = \frac{1}{2}mv^2, \qquad K_{\text{rot}} = \frac{1}{2}I\omega^2Ktrans​=21​mv2,Krot​=21​Iω2

Also, for rolling without slipping, ω=vR\omega = \frac{v}{R}ω=Rv​


  1. For the solid sphere

Moment of inertia of a solid sphere about its center: Isphere=25mR2I_{\text{sphere}} = \frac{2}{5}mR^2Isphere​=52​mR2

Thus, Krot=12⋅25mR2⋅v2R2=15mv2K_{\text{rot}} = \frac{1}{2}\cdot \frac{2}{5}mR^2 \cdot \frac{v^2}{R^2} = \frac{1}{5}mv^2Krot​=21​⋅52​mR2⋅R2v2​=51​mv2

So total initial kinetic energy is Ksphere=12mv2+15mv2=710mv2K_{\text{sphere}} = \frac{1}{2}mv^2 + \frac{1}{5}mv^2 = \frac{7}{10}mv^2Ksphere​=21​mv2+51​mv2=107​mv2

At maximum height h1h_1h1​, mgh1=710mv2mgh_1 = \frac{7}{10}mv^2mgh1​=107​mv2

Hence, h1=7v210gh_1 = \frac{7v^2}{10g}h1​=10g7v2​


  1. For the hollow cylinder

For a hollow cylinder (ring), Icyl=mR2I_{\text{cyl}} = mR^2Icyl​=mR2

Thus, Krot=12mR2⋅v2R2=12mv2K_{\text{rot}} = \frac{1}{2}mR^2 \cdot \frac{v^2}{R^2} = \frac{1}{2}mv^2Krot​=21​mR2⋅R2v2​=21​mv2

So total initial kinetic energy is Kcyl=12mv2+12mv2=mv2K_{\text{cyl}} = \frac{1}{2}mv^2 + \frac{1}{2}mv^2 = mv^2Kcyl​=21​mv2+21​mv2=mv2

At maximum height h2h_2h2​, mgh2=mv2mgh_2 = mv^2mgh2​=mv2

Hence, h2=v2gh_2 = \frac{v^2}{g}h2​=gv2​


  1. Find the ratio

h1:h2=7v210g:v2g=710:1=7:10h_1:h_2 = \frac{7v^2}{10g} : \frac{v^2}{g} = \frac{7}{10}:1 = 7:10h1​:h2​=10g7v2​:gv2​=107​:1=7:10

Given, h1:h2=n10h_1:h_2 = \frac{n}{10}h1​:h2​=10n​

So, n10=710⇒n=7\frac{n}{10} = \frac{7}{10} \Rightarrow n=710n​=107​⇒n=7


  1. Final answer

7\boxed{7}7​

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