$2\mathrm{.}20.$ A $120mm$ disc rotates on a table separated by an oil film of $1\mathrm{.}8mm$ thickness. Find the viscosity of oil if the torque required to rotate the disc at $60$ r$.$p$.$m is $3\mathrm{.}6{10}^{-4}Nm.$ Assume the velocity gradient in the oil film to be linear.

$T={\displaystyle \underset{0}{\overset{R}{}}}\frac{2r^{3}dr}{t}=\frac{2}{t}{\displaystyle \underset{0}{\overset{R}{}}}{r}^{3}dr=\frac{R^4}{2t}$

$T=\frac{R^4}{?t}$