Q.101
Assumptions and Limitations of Stokes’ Law
(A) . Particles must be spherical, smooth and rigid. But, soil colloidal particles are plateshaped and fall slower than spherical
particles of the same mass.
(B) . Size of particles must be large compared to the size of molecules of the liquid, so
that the medium can be considered homogeneous, i.e. no Brownian movement occurs.
(C) . Fall must be unhindered. Particles falling very near the wall of the vessel (0.1 mm.
distance) are slowed down in their descent. Many fast falling particles may drag finer
particles down along with them (concentration up to 3% solids)
(D) . Particles must be of uniform density and the suspension must be still without any
turbulence
Choose the correct answer from the options given below:
- Stokes' Law Assumptions in Soil Mechanical Analysis:
- Particles must be rigid, smooth, and spherical; soil clay platelets fall slightly slower than equivalent spherical masses.
- Particle size must be significantly larger than water molecules ($\gt 0.0002\text{ mm}$) to prevent Brownian motion.
- Settling must be unhindered in an infinite fluid medium without wall effects ($\gt 0.1\text{ mm}$ clearance) and particle concentration below 2–3% solids.
- Particles must have uniform density (conventionally assumed as $2.65\text{ g/cm}^3$) in a quiescent, non-turbulent fluid.
- Conclusion: Statements (A), (B), (C), and (D) are all valid assumptions and limitations. (Correct answer: Option 3)
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