Suppose the particle of mass 'm' is moving parallel to x-axis with constant velocity v and starting from B, where OB= b(see Fig.) At the time 't' let the particle be at p, where x = OA = BP = vt, y = b, z = 0.
Components of velocity are:
i.e., angular momentum has magnitude m vb and is directed along negative z-axis. Thus, angular momentum remains constant.
