Text books
Book – George C. King, 2009. Vibrations and Waves. UK, Wiley publications (pp. 78–81).
Internet
http://www.theorphys.science.ru.nl/people/fasolino/sub_java/pendula/doublependul-en.shtmlJava apps/apps
University of Colorado, B. (n.d.). Masses & Springs. Retrieved from PhET Interactive Simulations:https://phet.colorado.edu/en/simulation/mass-spring-labA
Videos