Consider the single-degree-of-freedom system shown in Figure 9.8. The excitation force due to the rotating unbalance is meω2sin(ωt). Assume that the system has the following parameters: = 5 kg, = 100 kg, = 0.1 m, = 5,000 N/m, and = 200 N・s/m.

a. Determine the force transmitted to the support when the system runs at the rotating speed ω = 1.5ωn.

b. Determine the force transmissibility.

c. The spring is replaced to decrease the force transmissibility at the same excitation

frequency to 20%. Determine the stiffness of the new spring.