Two masses of 'm/2' each are connected by a massless rigid rod of length 'l' and placed on a horizontal frictionless table. A third mass 'm' moving with velocity 'v' on the table strikes one of the masses in a head on elastic collision. Line of action is perpendicular to the length of the rod. Conservation of linear momentum and equation of elastic collision yields the velocity of the two colliding masses (as v/3 and 4v/3). Now, calculate initial and final angular momentum relative to the centre of mass of the masses attached at the ends of the rod. They will be different (mvl and mvl/6). (Here consider that the COM aquires a velocity of 2v/3). Where is the flaw since no external torque acts on the system in the frame of the COM of the two masses also? Try !!!