This demonstration illustrates resonance in a very simple setting - the RLC circuit. There are sliders that you can use to set the frequency, w (in radians per second), and amplitude, A (in volts), of the input voltage as well as the resistance R (in ohms), inductance, L (in henries), and capacitance, C (in microfarads), of the circuit elements. Each time you change any of these settings, you should let the transients damp out before drawing any conclusions about the relationship between the input and output voltages. The magnitudes and times of the most recent maximum input and output voltages are displayed to help you tell whether or not the transients have damped out.
The demonstration below is an applet. Google Chrome, Firefox and Microsoft Edge no longer execute applets because of security issues with NPAPI plugins. Some browsers that still play applets are Internet Explorer, Safari and the Pale Moon browser [ windows, linux, Mac ].