A 2 cm wide gap between two vertical plane surfaces is filled with an...
The shear stresses on two sides of the plate are
τ1 = μ du / dy = μ V / t1 and τ2 = μ V / t2
Drag force or viscous resistance against the motion of plate,
F = (μ V / t1+ μ V / t2)A
= μAV [ 1 / t2 +1 / t 2]
Since the plate is placed midway in the gap,
t1 = t2 and therefore,
F = 2μAV / t
Where t = t1 = t2 = 2 − 0.2 / 2 = 0.9 cm = 0.009 m
∴ F = 2 × 2.5(1.25 × 1.25) × 0.12 / 0.009
= 104.17 N
Upthrust or buoyant force on the plate
= specific weight of oil × volume of oil displaced
= (0.85 × 9810) × (1.25 × 1.25 × 0.002)
= 26.06 N
Effective weight of the plate = 30 − 26.06 = 3.94 N
∴ Total force required to lift the plate at the given velocity,
= 104.17 + 3.94 = 108.11 N