Question

what is the law of moment of inertia of a certain cross section about its axis ?

Answer 1

**law of moment of inertia of a certain cross section about its axis**

not sure this makes total sense to me, but you may be referring to the parallel axis theorem:\[I = I_{cm} + mr^2\]

or you may be referring to the equivalent of Newton's laws for rotation, especially the 2nd law:
\[\vec \tau = I \vec \alpha\].

Torque = Rotational Inertia times angular acceleration