Physics Fluid Dynamics PMDC Conceptual Practice
PMDC Verified Question 24 of 37
Which of the following equations correctly represents Stokes' Law for the drag force acting on a sphere moving slowly through a viscous fluid?
A
\( F = 6 \pi \eta r v^{-1} \)
B
\( F = 6 \pi \eta r^2 v \)
C
\( F = 6 \pi^2 \eta r^3 v \)
D
\( F = 6 \pi \eta r v \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: \( F = 6 \pi \eta r v \)
Concept:

Stokes' law determines the frictional force restricting a spherical body falling through a uniform continuous viscous medium at low Reynolds numbers.

Formula:

$$ F = 6 \pi \eta r v $$

Solution:

  • \( \eta \) is the dynamic viscosity of the fluid.
  • \( r \) is the radius of the sphere.
  • \( v \) is the velocity of the sphere.
  • The drag force is strictly directly proportional to all three variables at low speeds.


Why other options are incorrect:

Option A falsely suggests an inverse relation with velocity. Options B and C use squared or cubed radii, which occur in other contexts (like cross-sectional area or volume) but are incorrect for Stokes' drag.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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