Problem 1.187

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 \(\hat{O}\) (Fig. 1.48\() .\) The angle between the motion direction of the disc and the normal of the wall is equal to \(\alpha .\) 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 \(O^{\prime}\) which is located in the plane of the disc's motion at the distance \(l\) from the point \(O\)