There are three springs all driven by external force with different frequency.
The resonance frequency is determined by \omega_0=\sqrt{k/m} where k is the spring constant, m is the mass (k,m can be adjusted with slider). 
When sync check box is checked:
The driven frequency for left spring is ratio*w_0,
The driven frequency for center spring is w_0,
The driven frequency for right spring is ratio/w_0.

You can find the phase between oscillation and external force.
A damping force is introduced to prevent the amplitude become infinity.

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