SSC JE Mechanical Engineering 2013 Paper

For shafts transmitting torques.
Shear stress is given by $\mathit{\tau }=\frac{\mathit{T}\times \mathit{r}}{\mathit{J}}$
r = Radius of shaft
J = Polar moment of Inertia
T =Torque transmitted
τ = Shear stress maximum
$\frac{\mathit{J}}{\mathit{r}}={\mathit{t}}_{\mathit{P}}$ → polar section modulus