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Rotational Motion question

2023 · 13 Apr · Shift 1 · Q71
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  5. /2023 · 13 Apr · Shift 1 · Q71

Rotational Motion question

2023 · 13 Apr · Shift 1 · Q71

JEE MainPhysicsRotational MotionNumerical+4 / −1
A solid sphere is rolling on a horizontal plane without slipping. If the ratio of angular momentum about axis of rotation of the sphere to the total energy of moving sphere is π:22\pi: 22π:22 then, the value of its angular speed will be ‾\underline{\hspace{2cm}}​rad/s\mathrm{rad} / \mathrm{s}rad/s.
Numerical answer
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Correct answer: 4

  1. Let the mass and radius of the solid sphere be mmm and RRR.

  2. Since the sphere is rolling without slipping, v=ωRv = \omega Rv=ωR where vvv is the speed of the centre of mass and ω\omegaω is the angular speed.

  3. Angular momentum about the axis of rotation: For a solid sphere, moment of inertia about its own axis is I=25mR2I = \frac{2}{5}mR^2I=52​mR2 So angular momentum about the axis of rotation is L=Iω=25mR2ωL = I\omega = \frac{2}{5}mR^2\omegaL=Iω=52​mR2ω

  4. Total energy of the moving sphere: This is the sum of translational and rotational kinetic energies. E=12mv2+12Iω2E = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2E=21​mv2+21​Iω2 Using v=ωRv=\omega Rv=ωR and I=25mR2I=\frac{2}{5}mR^2I=52​mR2, E=12m(ωR)2+12⋅25mR2ω2E = \frac{1}{2}m(\omega R)^2 + \frac{1}{2}\cdot \frac{2}{5}mR^2\omega^2E=21​m(ωR)2+21​⋅52​mR2ω2 E=12mR2ω2+15mR2ω2E = \frac{1}{2}mR^2\omega^2 + \frac{1}{5}mR^2\omega^2E=21​mR2ω2+51​mR2ω2 E=710mR2ω2E = \frac{7}{10}mR^2\omega^2E=107​mR2ω2

  5. Given ratio: L:E=π:22L : E = \pi : 22L:E=π:22 So, LE=π22\frac{L}{E} = \frac{\pi}{22}EL​=22π​ Substitute LLL and EEE: 25mR2ω710mR2ω2=π22\frac{\frac{2}{5}mR^2\omega}{\frac{7}{10}mR^2\omega^2} = \frac{\pi}{22}107​mR2ω252​mR2ω​=22π​ Cancel mR2mR^2mR2: 25⋅107⋅1ω=π22\frac{2}{5}\cdot \frac{10}{7}\cdot \frac{1}{\omega} = \frac{\pi}{22}52​⋅710​⋅ω1​=22π​ 47ω=π22\frac{4}{7\omega} = \frac{\pi}{22}7ω4​=22π​

  6. Solve for ω\omegaω: ω=4⋅227π=887π\omega = \frac{4\cdot 22}{7\pi} = \frac{88}{7\pi}ω=7π4⋅22​=7π88​ Using π=227\pi = \frac{22}{7}π=722​, ω=887⋅227=8822=4\omega = \frac{88}{7\cdot \frac{22}{7}} = \frac{88}{22} = 4ω=7⋅722​88​=2288​=4

Therefore, 4 rad/s\boxed{4\ \text{rad/s}}4 rad/s​

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