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Question #42050

Difficulty: 8/10 | 🟢 Free

The Question

A uniform rigid door of mass M=6 kgM = 6 \text{ kg} and width L=2 mL = 2 \text{ m} is hinged at one edge. At t=0t=0, a sensor triggers an automated motor which applies a torque Γ(t)=12t2 N⋅m\Gamma(t) = 12t^2 \text{ N}\cdot\text{m}. Due to a reaction time gap of the release latch, the door remains locked and stationary until t=1 st = 1 \text{ s}. At exactly t=1 st = 1 \text{ s}, the latch instantaneously disengages. Assuming no friction in the hinge, what is the work done by the motor on the door during the interval t=0t=0 to t=3 st=3 \text{ s}?

Options

  • A.

    676 J676 \text{ J}

  • B.

    729 J729 \text{ J}

  • C.

    104 J104 \text{ J}

  • D.

    64 J64 \text{ J}

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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

The Reaction Time Gap|Instantaneous Nature|Work-Energy Theorem for Rotation

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to correctly apply the Work-Energy Theorem in the presence of a reaction time gap and a time-dependent force.

A common trap is blindly integrating the angular acceleration from t=0t=0 (ignoring the boundary condition ω=0\omega=0 at t=1t=1), which yields an incorrect angular velocity of 13.5 rad/s13.5 \text{ rad/s} and a kinetic energy of 729 J729 \text{ J}.

Another mistake is calculating work as the time integral of torque (∫Γdt\int \Gamma dt) rather than the angular displacement integral (∫Γdθ\int \Gamma d\theta), leading to 104 J104 \text{ J}.

The problem enforces the understanding that work is strictly zero when displacement is zero, despite a large applied force, and that kinematic integration must respect the physical release time.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterMotion in a Straight Line - Class 11 Part 1
ConceptThe Reaction Time Gap
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree