A plate of area 0.08m2 moves at 0.4m/s over a 0.25mm thick oil film (η\=0.034Pa s). What force is required?
Viscous force: F = ηvAl. η = 0.034Pa s, v = 0.4m/s, A = 0.08m2, l = 0.25×10−3m. F = 0.034×0.4×0.080.25×10−3 = 0.034×128 = 4.352N. As per NCERT, applying relevant law/formula with correct units and sign convention leads to 4.35 N. This satisfies dimensional consistency and physical conditions given, so option C is scientifically correct.
Ref: NCERT Class 11 Physics, Thermal Properties and Gravitation.