All of those topics are welcome in the Advanced Applied Mathematics forum. You can see from its subtitle that Mathematical Physics is included there, which would incorporate all of the mechanics you mentioned.
I agree completely with Ackbach. We are first and foremost a math message board. Adding some physics subforums would indicate to users that we encourage physics questions and we are capable of answering such questions. Keeping our scope limited helps us in many ways, even though this seems counter intuitive at first.