A counter-rotating eccentric mass exciter consisting of two rotating 3.5-oz weights describing circles of radius r at

Question:

A counter-rotating eccentric mass exciter consisting of two rotating 3.5-oz weights describing circles of radius r at the same speed but in opposite senses is placed on a machine element to induce a steady-state vibration of the element. The total weight of the system is 600 lb, the constant of each spring is k = 3400 lb/in., and the rotational speed of the exciter is 1200 rpm. Knowing that the amplitude of the total fluctuating force exerted on the foundation is 36 lb, determine the radius r.