Physics Fluid Dynamics PMDC Conceptual Practice
PMDC Verified Question 28 of 37
The viscous drag force acting on a small, perfectly spherical body moving at very low speeds through a fluid is directly proportional to:
A
The square root of its speed (\( \sqrt{v} \))
B
The inverse square root of its speed (\( 1/\sqrt{v} \))
C
Its speed (\( v \))
D
The square of its speed (\( v^2 \))
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: Its speed (\( v \))
Concept:

Stokes' law characterizes the frictional drag behavior in the laminar flow regime (low velocity).

Formula:

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

Solution:

  • For a given object, radius \( r \) and fluid viscosity \( \eta \) are constants.
  • Thus, the formula simplifies to \( F_s \propto v \).
  • The drag force is directly and linearly proportional to the speed \( v \).


Why other options are incorrect:

At much higher velocities where turbulence dominates, drag becomes proportional to \( v^2 \) (Newtonian drag), but the prompt strictly specifies 'very low speeds' (Stokes' drag regime).

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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