Two masses $M_1$ of 2 kg and $M_2$ of 4 kg are connected by a light inextensible string. The string passes over a smooth pulley at the top of a rough plane inclined at $45^{\circ }$ to the horizontal. $M_1$ rests on the plane and $M_2$ hangs freely. The system is released from rest and $M_1$ accelerates up the plane at 2.5 ms^-2.