Why are classical mechanics questions moved to the physics exchange?
There is a classical-mechanics tag here.
I don't know much about mechanics, but we can generalize what Bye_World says in a comment above to all applied math questions: deciding where to ask is a matter of weight/balance of math vs. application context.
There are fairly simple mechanics problems (the forced pendulum comes to my mind; see  ), where the mathematical difficulty/issues far outweigh the fairly simple mechanics one needs to understand to state problem. Not surprisingly, most in-depth work on such problems was published by mathematicians. (To give you an example of [fairly hard] physics-related question that I think was correctly asked here see How to solve 29 coupled quadratic equations?)
On the other hand, if someone is asking a question where they are mostly confused about physical laws and how to apply them, migration is the best choice. There were actually some questions like that that I commented on here, but they appear to have been deleted, so (not having anywhere near enough rep) I can't find & link to them anymore. One [perhaps slighly below] borderline question is Applying linear algebra to solve a problem in mechanical equilibrium. Had it been asked on the physics site it might have received an answer by now.