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Question #42428

Difficulty: 8/10 | 🟢 Free

The Question

A block of mass M=18.75 kgM = 18.75 \text{ kg} rests on a smooth horizontal table. A long, light string of linear mass density μ=0.01 kg/m\mu = 0.01 \text{ kg/m} is attached to the block, passes around a light movable pulley, and has its other end fixed to a vertical wall. The string segments are horizontal. At t=0t=0, the movable pulley is pulled away from the wall with a time-varying acceleration ap=6t m/s2a_p = 6t \text{ m/s}^2. At the exact same instant t=0t=0, a transverse wave pulse is generated at the block. The initial distance between the block and the pulley is L0=101 mL_0 = 101 \text{ m}. Assuming the tension is uniform along the string at any given instant, find the time t0t_0 when the pulse reaches the pulley.

Options

  • A.

    1 s1 \text{ s}

  • B.

    (1.01)2/3 s(1.01)^{2/3} \text{ s}

  • C.

    (101502)2/3 s\left(\frac{101}{50\sqrt{2}}\right)^{2/3} \text{ s}

  • D.

    2 s2 \text{ s}

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Variable_Constraint

Develop pure symbolic logic skills by mastering algebraic constraints without relying on numerical data.

Concepts Behind This Question

Constraint Relations via String Length|String Tension and Wave Speed|Partial Differentiation

Why This Question Is Tricky

Common Mistakes

This problem tests the synthesis of non-inertial wave dynamics and kinematic constraints.

The primary trap is the assumption of a quasi-static or constant wave speed.

Students must first use constraint relations to find the block's acceleration, which dictates a time-varying tension T(t)T(t) and thus a modulating wave speed c(t)c(t).

The secondary, more fatal trap is integrating c(t)c(t) and equating it to the initial length L0L_0.

Students must realize that the string itself acts as a moving medium (vbv_b) and the target boundary (the pulley) is also accelerating (vpv_p).

The correct formulation requires transforming the wave speed into the lab frame and equating the absolute dynamic positions of the pulse and the pulley.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterLaws of Motion - Class 11 Part 1
ConceptConstraint Relations via String Length
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree