Hi. This is a simple one (I think): We all know a swing, or simple mass on a spring, moves in harmonic motion, and has a specific resonative frequency. We also know that the RCL circuit (which includes a coil and a capacitor) reacts similarly to an external oscillator, also with a specific frequency which gives the highest output. Both systems are equivalents, right? So my question is: If we decide the mechanical velocity is equivalent to the current, and the spring equivalent to the capacitor, what is the mechanical equivalent of a coil?