Two particles, \(P\) and \(Q\), have masses \(3 m\) and \(m\) respectively. They are moving in opposite directions towards each other along the same straight line on a smooth horizontal plane and collide directly. The speeds of \(P\) and \(Q\) immediately before the collision are \(2 u\) and \(4 u\) respectively. The magnitude of the impulse received by each particle in the collision is \(\frac { 21 m u } { 4 }\).