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Rotational Motion question

2023 · 11 Apr · Shift 1 · Q72
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  5. /2023 · 11 Apr · Shift 1 · Q72

Rotational Motion question

2023 · 11 Apr · Shift 1 · Q72

JEE MainPhysicsRotational MotionNumerical+4 / −1
A solid sphere of mass 500 g500 \mathrm{~g}500 g and radius 5 cm5 \mathrm{~cm}5 cm is rotated about one of its diameter with angular speed of 10 rad s−110 ~\mathrm{rad} ~\mathrm{s}^{-1}10 rad s−1. If the moment of inertia of the sphere about its tangent is x×10−2x \times 10^{-2}x×10−2 times its angular momentum about the diameter. Then the value of xxx will be ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 35

  1. Given data
  • Mass of sphere: m=500 g=0.5 kgm = 500\,\text{g} = 0.5\,\text{kg}m=500g=0.5kg
  • Radius: R=5 cm=0.05 mR = 5\,\text{cm} = 0.05\,\text{m}R=5cm=0.05m
  • Angular speed about diameter: ω=10 rad s−1\omega = 10\,\text{rad s}^{-1}ω=10rad s−1

We need to use:

  • Moment of inertia about a tangent
  • Angular momentum about the diameter

The statement says:

Itangent=x×10−2×LdiameterI_{\text{tangent}} = x \times 10^{-2} \times L_{\text{diameter}}Itangent​=x×10−2×Ldiameter​

where LdiameterL_{\text{diameter}}Ldiameter​ is the angular momentum about the diameter.


  1. Moment of inertia about the diameter

For a solid sphere about any diameter,

Idiameter=25mR2I_{\text{diameter}} = \frac{2}{5}mR^2Idiameter​=52​mR2

So,

Idiameter=25(0.5)(0.05)2I_{\text{diameter}} = \frac{2}{5}(0.5)(0.05)^2Idiameter​=52​(0.5)(0.05)2

=25(0.5)(0.0025)= \frac{2}{5}(0.5)(0.0025)=52​(0.5)(0.0025)

=0.0005 kg m2= 0.0005\,\text{kg m}^2=0.0005kg m2


  1. Angular momentum about the diameter

Ldiameter=Idiameter ωL_{\text{diameter}} = I_{\text{diameter}}\,\omegaLdiameter​=Idiameter​ω

Ldiameter=0.0005×10=0.005 kg m2/sL_{\text{diameter}} = 0.0005 \times 10 = 0.005\,\text{kg m}^2\text{/s}Ldiameter​=0.0005×10=0.005kg m2/s


  1. Moment of inertia about a tangent

Using parallel axis theorem,

Itangent=Idiameter+mR2I_{\text{tangent}} = I_{\text{diameter}} + mR^2Itangent​=Idiameter​+mR2

=25mR2+mR2=75mR2= \frac{2}{5}mR^2 + mR^2 = \frac{7}{5}mR^2=52​mR2+mR2=57​mR2

Now,

Itangent=75(0.5)(0.05)2I_{\text{tangent}} = \frac{7}{5}(0.5)(0.05)^2Itangent​=57​(0.5)(0.05)2

=75(0.5)(0.0025)= \frac{7}{5}(0.5)(0.0025)=57​(0.5)(0.0025)

=0.00175 kg m2= 0.00175\,\text{kg m}^2=0.00175kg m2


  1. Relate with given expression

Given,

Itangent=x×10−2×LdiameterI_{\text{tangent}} = x \times 10^{-2} \times L_{\text{diameter}}Itangent​=x×10−2×Ldiameter​

Substitute values:

0.00175=x×10−2×0.0050.00175 = x \times 10^{-2} \times 0.0050.00175=x×10−2×0.005

0.00175=x×0.000050.00175 = x \times 0.000050.00175=x×0.00005

x=0.001750.00005=35x = \frac{0.00175}{0.00005} = 35x=0.000050.00175​=35


  1. Final answer

x=35\boxed{x = 35}x=35​

The derived answer matches the stored correct answer.

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