6. Two particles, each of mass m, are connected by a light inextensible string of length 2l. Initially they lie on a smooth horizontal table at points A and B distant l apart. The particle at A is projected across the table with velocity u. Find the speed with which the second particle begins to move if the directions of u is. (a) along AB, (b) at an angle of 120 degree with AB, (c) perpendicular to AB. How to solve for the b and c part? What`s the usage of the 2 given lengths?