but as the line of normal force passes through the centre of mass,so the torque by normal force about that point must be zero.

please explain.

Q- particles of masses 1g,2g,3g...........,100g are kept at the marks 1cm,2cm,3cm,....................100cm respectively on a metre scale. find the moment of inertia of the system of particles about a perpendicular bisector of the metre scale.

 

please solve