Example 2: Spring-mass system

 
A mass suspended from a spring, displaced vertically and released to move freely to produce up and down motion

Run Model:
https://dl.dropboxusercontent.com/u/44365627/lookangEJSworkspace/export/ejss_model_SHMxvavertical/SHMxvavertical_Simulation.html

Extra Info:

http://weelookang.blogspot.sg/2014/02/ejss-vertical-spring-mass-model.html

Q1: under what conditions would this vertical spring mass system’s motion be not well modelled as a simple harmonic motion?

H1: the model assumes the restoring force by the spring is F = -ky, it is always correct for any magnitude of oscillation y in real springs. Is it always valid in the real spring motion when the spring is hyper extending beyond the linear limits of Hooke's Law ?