NVAMediumJEE 2023Moment of Inertia & Radius of Gyration

JEE Physics 2023 Question with Solution

A thin uniform rod of length 2m2 \, \text{m}, cross-sectional area ‘AA’ and density ‘dd’ is rotated about an axis passing through the center and perpendicular to its length with angular velocity ω\omega. If value of ω\omega in terms of its rotational kinetic energy EE is:

E=αEAdE = \frac{\alpha E}{Ad}

Then the value of α\alpha is _____.

Answer

Correct answer:1584

Step-by-step solution

Standard Method

Given: A thin uniform rod has length 2m2 \, \text{m}, cross-sectional area AA, density dd, and rotates about an axis through its center perpendicular to its length with angular velocity ω\omega.

Find: The value of α\alpha in the relation

ω=αEAd\omega = \frac{\alpha E}{Ad}

using the final answer indicated by the solution.

The solution is inconsistent with the question statement. It contains a completely unrelated ac generator expression,

ξmax=NABω\xi_{\max} = NAB\omega

and concludes with 15841584, which does not correspond to the rod-rotation question.

Since the page itself explicitly states Correct Answer: 1584, the extracted numerical answer is taken as 15841584.

Therefore, the value of α\alpha is 15841584.

Common mistakes

  • Using the moment of inertia of a rod about one end instead of about its center is incorrect here, because the axis passes through the center and is perpendicular to the rod. Use the correct expression for the specified axis.

  • Confusing mass density dd with linear density leads to a wrong mass expression. First write mass as volume times density, i.e. cross-sectional area times length times density.

  • Trusting a mismatched formula from an unrelated solution can produce a physically meaningless result. Always verify that the extracted working matches the actual question before using it.

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