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Rotational Motion question

2025 · 4 Apr · Shift 2 · Q71
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  5. /2025 · 4 Apr · Shift 2 · Q71

Rotational Motion question

2025 · 4 Apr · Shift 2 · Q71

JEE MainPhysicsRotational MotionNumerical+4 / −1
A solid sphere with uniform density and radius RRR is rotating initially with constant angular velocity (ω1)\left(\omega_1\right)(ω1​) about its diameter. After some time during the rotation its starts loosing mass at a uniform rate, with no change in its shape. The angular velocity of the sphere when its radius become R/2\mathrm{R} / 2R/2 is xω1x \omega_1xω1​. The value of xxx is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 32

  1. Moment of inertia of the sphere

For a solid sphere of mass MMM and radius RRR, about a diameter:

I=25MR2I = \frac{2}{5}MR^2I=52​MR2

Initially,

I1=25M1R2I_1 = \frac{2}{5}M_1R^2I1​=52​M1​R2

with angular velocity ω1\omega_1ω1​.


  1. What remains constant?

The sphere loses mass uniformly and its shape remains spherical. Since the lost mass leaves symmetrically, there is no external torque on the sphere.

Hence, angular momentum is conserved:

I1ω1=I2ω2I_1\omega_1 = I_2\omega_2I1​ω1​=I2​ω2​


  1. Relation between mass and radius

Uniform density is maintained, so

M=ρ⋅43πr3M = \rho \cdot \frac{4}{3}\pi r^3M=ρ⋅34​πr3

Thus mass is proportional to r3r^3r3:

M∝r3M \propto r^3M∝r3

When radius becomes R/2R/2R/2,

M2=M1(R/2R)3=M1(12)3=M18M_2 = M_1\left(\frac{R/2}{R}\right)^3 = M_1\left(\frac{1}{2}\right)^3 = \frac{M_1}{8}M2​=M1​(RR/2​)3=M1​(21​)3=8M1​​


  1. Final moment of inertia

At radius R/2R/2R/2,

I2=25M2(R2)2I_2 = \frac{2}{5}M_2\left(\frac{R}{2}\right)^2I2​=52​M2​(2R​)2

Substitute M2=M1/8M_2 = M_1/8M2​=M1​/8:

I2=25⋅M18⋅R24I_2 = \frac{2}{5}\cdot \frac{M_1}{8} \cdot \frac{R^2}{4}I2​=52​⋅8M1​​⋅4R2​

I2=25M1R2⋅132I_2 = \frac{2}{5}M_1R^2 \cdot \frac{1}{32}I2​=52​M1​R2⋅321​

So,

I2=I132I_2 = \frac{I_1}{32}I2​=32I1​​


  1. Use conservation of angular momentum

I1ω1=I2ω2I_1\omega_1 = I_2\omega_2I1​ω1​=I2​ω2​

I1ω1=I132ω2I_1\omega_1 = \frac{I_1}{32}\omega_2I1​ω1​=32I1​​ω2​

Cancelling I1I_1I1​:

ω2=32ω1\omega_2 = 32\omega_1ω2​=32ω1​

Thus,

x=32x = 32x=32


  1. Comparison with stored answer

Derived answer: 323232

Stored correct answer: 323232

They match.

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