Resonance in a string
The length of the rope can be varied by dragging the stand. The tension in the string can hence be changed.
When the ruler is shown, a yellow vertical line will appear when the mouse is pressed inside it.
Resonance in Mechanics:Force Object to Oscillate through the Action of Spring
Resonance in Electricity: RLC Series Resonance, RLC Parallel Resonance
The equation solved for this applet (pdf)