A stationary object of mass \(4m\) is located on a horizontal, frictionless surface. The object explodes and splits into two fragments of masses \(m\) and \(3m\) that move in opposite directions along a straight line. If the total kinetic energy of the two fragments immediately after the explosion is \(K\), which of the following is an expression for the magnitude of the momentum of the fragment of mass \(m\)?