Which of the following statements regarding the frictional properties of agricultural produce on structural surfaces are CORRECT?
Options:
(A) The coefficient of external friction of grains against galvanized steel is typically lower than that against unplaned plywood.
(B) The kinetic (dynamic) coefficient of friction is consistently lower than the static coefficient of friction for grain sliding on metal sheets.
(C) Within normal harvesting moisture ranges ($12\%\text{ to }22\%\text{ wb}$), the coefficient of friction of paddy on steel surfaces increases with an increase in moisture content.
(D) The angle of internal friction of grains is always strictly independent of the normal consolidating stress.
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
Analysis of statements:
- Option A is CORRECT: Galvanized steel has a much smoother surface finish and lower roughness ($R_a$) than unplaned plywood, resulting in a lower coefficient of friction.
- Option B is CORRECT: By the fundamental laws of friction, $\mu_k < \mu_s$ because dynamic shearing of microscopic asperities requires less force than breaking static adhesive junctions.
- Option C is CORRECT: Higher moisture softens the grain surface and creates capillary liquid bridges (meniscus forces) between the kernel and the metal sheet, increasing adhesive friction.
- Option D is INCORRECT: At very high consolidating pressures in deep silos, granular rearrangement causes the Mohr-Coulomb failure envelope to exhibit curvature, meaning the internal friction angle is not strictly constant.