A passenger is travelling in a train moving at 72 ${\rm{km}}{{\rm{h}}^{ - 1}}.$. His suitcase is kept on the berth. The driver of the train applies brakes such that the speed of the train decreases at a constant rate of 36 ${\rm{km}}{{\rm{h}}^{ - 1}}.$ in 5 s. What should be the minimum coefficient of friction between the suitcase and the berth if the suitcase is not the slide during retardation of the train?