Masses are arranged in ascending order of mass from left to right and suspended between two springs vertically from a fixed wooden frame. By pushing up and down on a plastic lever at the bottom of this frame, the frame can be vertically shaken. If pushing down on the lever 18 times in a 10-second interval causes the middle mass to swing vertically with a large amplitude, how frequently would you have to push down on the lever to excite each of the other masses of the frame? You may assume that the leftmost mass has mass m, the next 2m, the middle 3m, the fourth 4m, and the fifth 5m.