Question

About which point can we conserve angular momentum

Answer 1

When we have a collision of two rigid bodies: about what point is angular momentum conserved? I had a specific example with a point mass colliding with the end of a rod and noticed that only if I calculated the conservation about the total centre of mass did I get the correct answer for the angular velocity after collision. However, I do not understand the physics behind why angular momentum is only conserved about certain points and would be grateful for an explanation. 

Answer 2

..If no net external torque acts on a system, the total angular momentum of the system remains constant. This statement describes the conservation of angular momentum. It is the third of the major conservation laws encountered in mechanics (along with the conservation of energy and of linear momentum).