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Question

A cubical block of side 'a' and density 'ρ' slides over a fixed inclined plane with constant velocity 'v'. There is a thin film of viscous fluid of thickness 't' between the plane and the block. Then the coefficient of viscosity of the thin film will be (Acceleration due to gravity is g):

A
η=ρagtsinθV
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B
ρagt2sinθV
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C
Vρagtsinθ
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D
none of these
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Solution

The correct option is A η=ρagtsinθV
Since the block moves with constant velocity, net force F on the block is zero.
The forces acting on the block are:
mgsinθ=a3ρgsinθg ....(1) downward along the inclined plane
Viscous force Fviscous=ηAdVdy=ηa2Vt .....(2) where
A=a2 is the are of a face of the cube which is in contact with the surface of the inclined plane.
η is the coefficient of viscosity
Vt is the velocity gradient
Equating (1) and (2),
a3ρgsinθg=ηa2Vt
η=tρgsinθaV

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