I wonder why RK for spring pendulum was closed (put on hold). What exactly is out of scope? Why it requires a professional consultant?
This question should've been tagged as homework probably but why out of scope? One can see that OP put some effort into it and probably he's not fluent with Mathematica yet. But it seems to be an ok student problem that would've been answered eventually with nice explaination of how Runge Kutta works and most likely featuring neat animation. And the RK is quite popular and common not to call the question too localized.
There are lot of question like "here's a link, do same in Mathematica" and these questions are not closed, sometime they attract great answers, like how-to-make-this-beautiful-animation
NDSolve
, so the question didn't add much to the other one that it linked to. For generally unattractive questions it's just sometimes hard to give a concise closing reason...