A mass of 500 kg is mounted on supports having a total stiffness of 100 kN/m and which provides viscous damping, the damping ratio being 0.4. The mass is constrained to move vertically and is subjected to a vertical damping force of the type Fcosθt. Determine the frequency at which resonance will occur and the maximum allowable value of ‘F' if the amplitude at resonance is to be restricted to 5 mm.