The steady state response is always harmonic, and has the same frequency as that of the forcing.
To see this mathematically, note that in each case the solution has the form . Recall that defines the frequency of the force, the frequency of base excitation, or the rotor angular velocity. Thus, the frequency of vibration is determined by the forcing, not by the properties of the spring-mass system. This is unlike the free vibration response.
You can also check this out using our applet. To switch off the transient solution, click on the checkbox labeled `show transient’. Then, try running the applet with different values for k, m and , as well as different forcing frequencies, to see what happens. As long as you have switched off the transient solution, the response will always be harmonic.