A disc A of mass m sliding over a smooth horizontal surface with velocity v experiences a perfectly elastic collision with a smooth stationary wall at a point O (Fig. 1.48). The angle between the motion direction of the disc and the normal of the wall is equal to a. Find:
(a) The points relative to which the angular momentum M of the disc remains constant in this process;
(b) The magnitude of the increment of the vector of the disc's angular momentum relative to the point O` which is located in the plane of the disc's motion at the distance l from the point O.

A a