See, this I tell always that what is happening to the ship as the wave passes by every point there is a pressure; each pressure you add up you get a force. We have say, told earlier that all the pressures every quantity is having same period at 10 second, because you know everything is sin omega T, that make sense because see when the wave is passing by this point there is a pressure, when same pressure will be repeating after 10 seconds. So, the net force also will be periodic with the same period, because you know see here there is a pressure points, now the same thing repeats exactly after 10 seconds, so it will, it is repetitive after only that period.