$($b$)$ A $20$ mm diameter soap bubble has an internal pressure $27\mathrm{.}576\frac{N}{m^2}$ greater than the outside atmospheric

pressure. Compute the surface tension of soap$-$air interface.

$($c$)$ Derive the torques generated by two concentric cylinders having radii $r_1$ and $r_2,$ and with the inter$-$cylinder

spacing filled with a viscous liquid of dynamic viscosity $.$ Cylinders are moving with different rotational

speeds of ${}_1$ and ${}_2.$