Policy change this week (Feb 16-22): we are conducting a week-long experiment in which community members rely more on their individual judgment to close homework questions, and this post should be considered a soft guideline rather than a rule.
It's not enough to just show your work and ask where you went wrong. If you just need someone to check your work, you can always seek out a friend, classmate, or teacher. As a rule of thumb, a good conceptual question should be useful even to someone who isn't looking at the problem you happen to be working on
What kinds of questions are considered homework questions?
A "homework question" is any question whose value lies in helping you understand the method by which the question can be solved, rather than getting the answer itself. This includes not just questions from actual homework assignments, but also self-study problems, puzzles, etc.
On the other hand, questions that come up in the course of doing a homework problem, but are separate from the main point of the problem, might not be considered homework questions. There's a bit of a judgment call to be made, depending on the context of the problem. If you're not sure, it's probably safer to treat your question as a homework question and later find out that it isn't, than the other way around.
Can I ask a homework question here?
Yes, but there are a couple of things you need to make sure of first.
As a general rule, we do not discourage homework questions, as long as they are related to physics. But do keep in mind that Physics Stack Exchange is not primarily a homework help site; it's a place to get specific conceptual physics questions answered. The list in the following section will help you ask questions about your homework in a way that fits in with the site's philosophy.
Also, make sure you know whether your learning institution (middle school, high school, college, etc.) and your teacher or professor allow you to consult other people, or to post the exact question on the internet. This is usually addressed by your institution's honor code or rules and regulations, and any specific class policies. You should ask your teacher whether asking a homework question here is appropriate before posting your question.
How should I ask a homework question on this website?
See if an existing question helps you
Check and see if someone has already asked a question that gives you the information you need. The search box at the top right corner of the page will be pretty useful here, but you can also try looking at tags that are relevant to your question.
If you find a prior question that seems relevant but doesn't clear up your confusion, mention it when you write your own question. That gives the people answering a better idea of what kinds of explanations don't work for you, and what might be more effective.
Ask about the specific concept that gives you trouble
We expect you to narrow down the problem to the particular concept that's giving you trouble and ask about that specifically. That produces a question that is more relevant to others who might be having the same problem, as well as probably more interesting to answer. As a side effect it shows that you're not just being lazy and trying to get us to do your work for you.
The best way to produce a focused, specific question is to show your work. Explain what you've been able to figure out so far and how you did it. Showing your work will help us gauge where you are having problems: if it is a technical thing near the end, a short to the point answer will suffice; if it is some fundamental problem with understanding the subject, somebody will then write a longer, more detailed response. It will also prevent people from spending a lot of time going over ground that you have already covered or understand well already.
It's not enough to just show your work and ask where you went wrong. If you just need someone to check your work, you can always seek out a friend, classmate, or teacher. As a rule of thumb, a good conceptual question should be useful even to someone who isn't looking at the problem you happen to be working on. Of course, it's still good to include the text of your problem, just in case (more on that a few paragraphs down).
Don't just copy the exact problem from your homework assignment or textbook. In particular, when you are asking for help, writing in imperative mode ("Show that...", "Compute...", or "Prove or find a counterexample: ...") is at the very least impolite: you are, after all, trying to ask a question, not give an assignment. It also turns many people off.
Reference the source
If you're asking about a specific homework problem from a textbook, include the book and the problem number, so that someone trying to answer the question can go look it up themselves if they need to. If you're asking about a specific problem from a custom assignment prepared by the instructor, it helps if you quote the complete text of the problem in your post. Again, this shouldn't be the entire content of the post - you still need to ask about the specific issue that's confusing you, in addition to quoting the problem - but you never know when the person answering might need additional information from the original problem.
Use the homework tag
Use the homework tag on your question, in addition to any other tags that identify the kind of physics involved. This lets answerers know that you're looking for an answer which explains the underlying concepts.
If you don't include the tag, someone will usually add it for you. If that happens, don't think that we're accusing you of lying about whether your question is from a homework assignment! The tag is used for any question in which the point is to learn the method you're using to solve it, rather than just to get the answer.
Why don't you provide a complete answer to homework questions?
This is pretty well covered by a discussion on the Math Stack Exchange site.
Providing an answer that doesn't help
a student learn is not in the
student's own best interest, and if a
solution complete enough to be copied
verbatim and handed in is given
immediately, it will encourage more
people to use the site as a free
homework service. In the spirit of
creating a lasting resource of
mathematical knowledge, you may come
back after a suitable amount of time
and edit your response to include a
more complete answer. Or even better,
the student can post his own correct
If someone posts an answer to a homework-type question that gives away a complete or near-complete solution, in most cases it will be temporarily deleted.
The rules for how to properly post homework questions can be a bit confusing, so here are some examples:
A good homework question states the problem clearly, shows an attempt to work through it, and identifies the specific issue that is giving the questioner trouble. These questions demonstrate that pretty well.
These homework questions don't show any effort put into solving the problem, and they are too specific to be of use to anybody except the person asking. That makes them inappropriate for this site.
Parts adapted from http://meta.math.stackexchange.com/questions/1803/how-to-ask-a-homework-question