Two masses M and m are connected with a (massless string that runs over a (massless, frictionless) pulley. Mass M is siting on ahorizontal surface, and mass m is hanging down from the string that goes over the pulley. lt will move down when the system isa lowed to move. Calculate the acce eration with which the masses are moving for the case of lal a frictioniess surface. and b) asurface with friction.