A flat plate 0.1 m2 area is pulled at 30 cm/s relative to another plate located at a distance of 0.01 cm from it, the fluid separating them being water with viscosity of 0.001 Ns/m2. The power required to maintain velocity will be

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ESE Mechanical 2020 Official Paper
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  1. 0.05 W
  2. 0.07 W
  3. 0.09 W
  4. 0.11 W

Answer (Detailed Solution Below)

Option 3 : 0.09 W
Free
ST 1: UPSC ESE (IES) Civil - Building Materials
20 Qs. 40 Marks 24 Mins

Detailed Solution

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Concept:

The tangential shear stress between two adjoining is proportional to velocity gradient in direction of perpendicular to the layer. This is known as law of viscosity.

Shear force acting on the moving plate is given by F = τ × A

Power required to maintain velocity U is given by P = F × U

Calculation:

Given μ = 0.001 Ns/m2, U = 0.3 m/s, Y = 0.01 cm = 0.01 × 10-2 m, A = 0.1 m2;

Velocity gradient or rate of shear strain is given by

τ = 0.001 × 3000 = 3 N/m2

F = 3 × 0.1 = 0.3 N,

P = 0.3 × 0.3 = 0.09 W

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