2. A ball of mass \(m\) is thrown vertically upwards from the ground with an initial speed \(u\). When the speed of the ball is \(v\), the magnitude of the air resistance is \(m k v\), where \(k\) is a positive constant. By modelling the ball as a particle, find, in terms of \(u , k\) and \(g\), the time taken for the ball to reach its greatest height.