I understand t.b.'s reasoning on why we should keep both the physics and mathematical-physics tags, but I notice that their wikis, and more specifically their excerpts, are sufficiently similar that it would be hard to tell which to use.
(physics): Questions related to mathematical physics which include application of mathematics to problems in physics and the development of mathematical methods suitable for such applications and for the formulation of physical theories
(mathematical-physics): "Mathematical physics consists of the application of mathematics to problems in physics and the development of mathematical methods suitable for such applications and for the formulation of physical theories." (from Journal of Mathematical Physics) This tag is intended for questions on methods used e.g. in quantum mechanics or general relativity at an advanced undergraduate or graduate level.
Both seem to be about mathematical physics and thus neither captures the most common type of physics question that is asked on math.SE. Most of physics problems that get posted here (rightly or not) are about finding the solution to a physics exercise. Example: Spring Calculation - find mass is about solving an ODE from a classical mechanics problem.
"Mathematical physics", on the other hand, doesn't just mean a physics problem with math in it. It has a (somewhat) specific meaning to physicists -- it refers to a branch of physics about making mathematical models of the real world or finding new mathematical methods and techniques for evaluating current models. An example of a mathematical physics problem that one might post here is "I've determined that [some equation] models [some physical system]. But I'm having trouble finding the spectrum of [this associated operator]".
In light of this, I think the (physics) tag excerpt should be changed to better distinguish it from the (mathematical-physics) tag and make it more applicable to these types of questions. I'd just do it myself, but having never edited a tag wiki, I don't want to screw it up. Plus I'd like your input.
P.S. Do we really need the (retagging) tag on this site?
Edit: I went ahead and suggested an edit for the (physics) tag wiki as well as the (classical mechanics) and (electromagnetism) wikis. I admit my edits are not ideal, but I think they're an improvement, nonetheless. Feel free to review it if you have 20K+ rep. Any feedback would be appreciated.