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Question #77082

Difficulty: 5/10 | 🟢 Free

The Question

A non-uniform rod of length LL (in m\text{m}) is placed on a smooth horizontal table. One end of the rod is attached to a vertical wire with torsional constant κ\kappa (in N⋅m⋅rad−1\text{N}\cdot\text{m}\cdot\text{rad}^{-1}). The linear mass density of the rod varies as λ(x)=βx\lambda(x) = \beta x, where xx is the distance from the attached end and β\beta is a positive constant (in kg⋅m−2\text{kg}\cdot\text{m}^{-2}). An angular displacement sensor measures the angle of the rod. The sensor has a constant positive zero error θ0\theta_0 (in rad\text{rad}), meaning it reads θ0\theta_0 when the wire is untwisted and exerts no torque. The rod is rotated until the sensor reads ϕ0\phi_0 (in rad\text{rad}, with ϕ0>θ0\phi_0 > \theta_0) and is released from rest. What is the maximum angular speed Ωmax\Omega_{max} (in rad⋅s−1\text{rad}\cdot\text{s}^{-1}) of the rod during its subsequent motion?

Options

  • A.

    2(ϕ0−θ0)L2κβ\frac{2(\phi_0 - \theta_0)}{L^2} \sqrt{\frac{\kappa}{\beta}}

  • B.

    6(ϕ0−θ0)L2κβ\frac{\sqrt{6}(\phi_0 - \theta_0)}{L^2} \sqrt{\frac{\kappa}{\beta}}

  • C.

    2ϕ0L2κβ\frac{2\phi_0}{L^2} \sqrt{\frac{\kappa}{\beta}}

  • D.

    3(ϕ0+θ0)2L2κβ\frac{3(\phi_0 + \theta_0)}{\sqrt{2}L^2} \sqrt{\frac{\kappa}{\beta}}

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Multi-Concept_Bridge

Master problems that require linking physics and math concepts to solve complex competitive questions.

Concepts Behind This Question

Zero Error Correction|Physical Pendulum Inertia|Torsional Oscillations|Integration

Why This Question Is Tricky

Examiner Levers

Dimensional Analysis

Tests if the student verifies units/dimensions before selecting.

Difficulty Boost : +2

Common Mistakes

This question tests the synthesis of experimental error analysis with rigid body dynamics.

Students often mechanically plug in the given initial angle (ϕ0\phi_0) as the amplitude, failing to recognize that the restoring torque τ=−κθ\tau = -\kappa\theta is governed by the true physical equilibrium, requiring the subtraction of the zero error θ0\theta_0.

Furthermore, the non-uniform mass distribution traps students who incorrectly assume a uniform rod of equivalent mass (I=ML2/3I = ML^2/3) or treat the mass as concentrated at the center of mass (I=Mxcm2I = Mx_{cm}^2).

The correct approach requires integrating the continuous mass distribution from first principles and applying the true physical amplitude to the simple harmonic motion velocity relation Ωmax=Aω\Omega_{max} = A\omega.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterUnits and Measurements - Class 11 Part 1
ConceptZero Error Correction in Instruments
Difficulty5
Cognitive Load6
Target Solve Time140 sec
AccessFree