"Off-topic" means more than just laterally off-topic. It also encompasses the concept of "too low-level" or "this flavor question doesn't belong here." So there are at least three broad categories of off-topicness:
- Lateral: The subject matter doesn't even belong here. Maybe it's engineering, maybe it's pure math, maybe it's about programming. It might have been inspired by physics, but in fact there's no more physics left to be done.
- Low level: By this I don't necessarily mean the subject matter itself is low level (as far as I know we don't really have a lower limit on content level), but rather that the question itself doesn't show any work. Saying "I don't get something, please make me understand" just doesn't cut it here. If a person is really that lost on, e.g., a homework assignment, the teacher should be sought out for extra explanation. Or a personal tutor may be needed. Or really just more work needs to be put in.1
- Wrong flavor: Off-topic might also mean that that the question asked is a question only in the technical syntactic sense, but that really it is being used as an open invitation for users to talk about a topic. Even if the subject is physics, any question that asks "hey, what do y'all think of
<cool thing>?" is decidedly off-topic.
This particular question suffers from the second point - it gives the setup, and then says something like "help, what do I do with this?" Yes we could give the answer, but nothing would be served by that. Even the OP wouldn't actually have learned anything; at best he could say he's memorized how to solve this particular problem and is one step closer to that stage in life so many people aim for where they never have to think for themselves again.
So in summary, "off-topic" should be interpreted more broadly than "not having to do with physics." Perhaps we could use different terms to distinguish these ideas, but eventually we would end up with an overabundance of specific "bad question" categories.
1 I'm a firm believer that students are never truly learning unless they are challenged to the point where they struggle with the concepts. After all, learning is about thinking in new ways, not about memorizing facts, formulas, or even procedures for solving problems.