Water is flowing on a horizontal fixed surface, such that its flow velocity varies with $y$ (vertical direction) as $v=k\left(\frac{2 y^2}{a^2}-\frac{y^3}{a^3}\right)$. If coefficient of viscosity for water is $\eta$, what will be shear stress between layers of water at $y=a$. 
  • A$\frac{\eta k}{a}$
  • B$\frac{\eta}{k a}$
  • C$\frac{\eta a}{k}$
  • D
    None of these
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