A metal plate of area 0.05m2 moves over a liquid film (η\=0.02Pa s) of thickness 0.2mm at 0.5m/s. What is the force requ
Viscous force: F = ηvAl. η = 0.02Pa s, v = 0.5m/s, A = 0.05m2, l = 0.2×10−3m. F = 0.02×0.5×0.050.2×10−3 = 0.02×125 = 2.5N. As per NCERT, applying relevant law/formula with correct units and sign convention leads to 2.5 N. This satisfies dimensional consistency and physical conditions given, so option B is scientifically correct.
Ref: NCERT Class 11 Physics, Thermal Properties and Gravitation.