Practice question
Question
A plate of area 0.07m2 moves at 0.3m/s over a 0.5mm thick water film (η\=1.0×10−3Pa s). What force is required?
Explanation
Viscous force: F = ηvAl. η = 1.0×10−3Pa s, v = 0.3m/s, A = 0.07m2, l = 0.5×10−3m. F = 1.0×10−3×0.3×0.070.5×10−3 = 1.0×10−3×42 = 0.042N. As per NCERT, applying relevant law/formula with correct units and sign convention leads to 0.042 N. This satisfies dimensional consistency and physical conditions given, so option B is scientifically correct.