The velocity of upper layer of water in a river is 36km h ^ - 1 ....

The viscous force formula is given by,

$F = - ηA\dfrac{d v}{d y}$

Therefore, shear stress $\dfrac{F}{A} = - η\dfrac{d v}{d y}$. Here negative sign tells us that the direction of force is opposite to the velocity gradient.

$⇒ \left|\dfrac{F}{A}\right| = η\dfrac{d v}{d y}$

Now $v = 36 \times \dfrac{5}{18} = 10m{s}^{- 1}$ and if the depth is $d$

$\dfrac{d v}{d y} = \dfrac{10}{d}$

$⇒ {10}^{- 3} = {10}^{- 2} \times \dfrac{10}{d} ⇒ d = 100m$