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Question #42022

Difficulty: 8/10 | 🟢 Free

The Question

A closed container is completely filled with a viscous incompressible fluid of density ρf\rho_f. The container accelerates horizontally along the positive x-axis with a constant acceleration a⃗=gi^\vec{a} = g\hat{i}, where gg is the acceleration due to gravity (acting along −gj^-g\hat{j}). A small spherical particle of mass mm, charge +q+q, and density ρp=2ρf\rho_p = 2\rho_f moves in the fluid. A uniform magnetic field B⃗=B0k^\vec{B} = B_0\hat{k} is present in the region. The fluid exerts a drag force F⃗D=−bv⃗\vec{F}_D = -b\vec{v}, where v⃗\vec{v} is the particle's velocity relative to the container. If the drag coefficient is b=qB0b = qB_0, what is the steady-state terminal velocity v⃗T\vec{v}_T of the particle relative to the container?

Options

  • A.

    −mg2bi^-\frac{mg}{2b}\hat{i}

  • B.

    −mgbi^-\frac{mg}{b}\hat{i}

  • C.

    −3mg4bi^+mg4bj^-\frac{3mg}{4b}\hat{i} + \frac{mg}{4b}\hat{j}

  • D.

    −mg2bj^-\frac{mg}{2b}\hat{j}

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Intuition_Breaker

Train your brain to resist "obvious" answers that are designed to deceive your intuition.

Concepts Behind This Question

The Directional Sign Convention|Lorentz Force Vector Direction|Accelerated Frame Hydrostatics

Why This Question Is Tricky

Common Mistakes

This question tests the simultaneous application of the directional sign convention for Lorentz forces and accelerated frame hydrostatics.

The primary trap is failing to recognize that the buoyancy force vector aligns with −g⃗eff-\vec{g}_{\text{eff}}, not just upwards (+j^+\hat{j}).

Students who ignore the pseudo-force contribution to the pressure gradient will incorrectly calculate the effective weight, leading to an erroneous terminal velocity vector.

Furthermore, setting up the vector equilibrium requires correctly resolving the cross product v⃗×B⃗\vec{v} \times \vec{B} and coupling the xx and yy velocity components through the drag force, testing rigorous non-conservative force vector summation.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterMotion in a Straight Line - Class 11 Part 1
ConceptThe Directional Sign Convention
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree