I have a question posted to MSE which is more about physics than math. If it remains on MSE, is it kosher to repost it here at the same time?
Generally speaking, cross-posting is not recommended without adapting the question to the audience intended. If your concern is the maths, then Math.SE is more appropriate & the question should be written to point out the interest in the maths. If your concern is there physics, then here would be the correct place with indications that it's about the physics.
That said, it seems to me that your question would probably be closed as a dupe of Contradiction between law of conservation of energy and law of conservation of momentum? (or one of the linked), were you to post it here.