Resonance on a String 

https://www.youtube.com/watch?v=C-Bn70PpbrM
Lesson 34 - Resonance - Forced Vibrations - Demonstrations in Physics
Professor Julius Sumner Miller

https://www.youtube.com/watch?v=lKT3wBIUFhA
Natural Frequency Vibration Test
Douglas Baxter


https://www.youtube.com/watch?v=dihQuwrf9yQ
A better description of resonance
Steve Mould


https://www.youtube.com/watch?v=cnH2ltfW48U
Modes on a String
geoff martin


https://www.youtube.com/watch?v=cnH2ltfW48U

https://www.youtube.com/watch?v=-gr7KmTOrx0