collapse | the above section is quoted for convenience from the full FAQ, below.

Physics Stack Exchange is for active researchers, academics and students of physics and astronomy. We welcome questions of all levels, but please stick to the following topics:

You can expect a range of answers from very basic to highly technical, depending on how you phrase your question.

Some kinds of questions should not be asked here:

• "Do my homework"-type physics questions
"A 4kg ball is traveling at 8m/s in the x direction, how do I find..."
Physics - Stack Exchange is not a homework help site. If you have a question about a homework problem, or any problem of an educational nature, narrow it down to the specific concept that is giving you trouble and ask about that. You can find more information about acceptable homework questions on our meta site.
• Pitches for your own personal theories or work
We deal with mainstream physics here. Anything that couldn't be published in a reputable journal is not appropriate on this site.
"Could a warp drive get you out of a black hole?"
Questions about physics of fictional worlds which are not sufficiently grounded in real physics are off topic here, but they may be on topic at Science Fiction & Fantasy.
• Computational questions
If your question is about writing a simulation or other program, and it concerns the algorithms, execution, or presentation of the results (rather than the underlying physics), it is probably more appropriate at Computational Science.

Physics relies heavily on math. Many posts include mathematical notation, written using LaTeX commands and delimited by dollar signs. If you see dollar signs with what appears to be gibberish between them, it just means that MathJax, the math rendering we use, isn't working properly for you.

Some basics for people new to LaTeX. Inline formulas are surrounded by single dollar signs ($f(x) = ax^2 + bx + c$ renders as $f(x) = ax^2 + bx + c$ ), and block formulas by double dollar signs so that $$x = \frac{-b \pm \sqrt{b^2 - 4ac} }{2a}$$ renders as

$$x = \frac{-b \pm \sqrt{b^2 - 4ac} }{2a} .$$

Greek letters are mostly written by simply spelling them after a backslash with capitalization indicated by first letter. That is \alpha \beta \Gamma \Delta renders as $\alpha \beta \Gamma \Delta$.

You should only ask practical, answerable questions based on actual problems that you face. Chatty, open-ended questions diminish the usefulness of our site and push other questions off the front page.

If your motivation for asking the question is “I would like to participate in a discussion about ______”, then you should not be asking here. However, if your motivation is “I would like others to explain ______ to me”, then you are probably OK. (Discussions are of course welcome in our real time web chat.)

To prevent your question from being flagged and possibly removed, avoid asking subjective questions where …

• your answer is provided along with the question, and you expect more answers: “I use ______ for ______, what do you use?”
• there is no actual problem to be solved: “I’m curious if other people feel like I do.”
• we are being asked an open-ended, hypothetical question: “What if ______ happened?”
• it is a rant disguised as a question: “______ sucks, am I right?”

(The above section was adapted from MetaFilter’s FAQ. For more detail, see six guidelines for great subjective questions.)

When you post a new question, other users will almost immediately see it and try to provide good answers. This often happens in a matter of minutes, so be sure to check back frequently when your question is still new for the best response.

Answers to your questions and comment replies to your posts will appear as a red indicator in your global inbox at the top left of every page; click it to view them.

Providing clarification promptly will help get you the best answers.

As you see new answers to your question, vote up the helpful ones by clicking the upward pointing arrow to the left of the answer.

Answers are normally sorted by vote score so the most highly voted answers float to the top. Other users will also vote on the answers to your question.

When you have decided which answer is the most helpful to you, mark it as the accepted answer by clicking on the check box outline to the left of the answer.

This lets other people know that you have received a good answer to your question. Doing this is helpful because it shows other people that you’re getting value from the community. (And if you don’t do this, people will often politely ask you to go back and accept answers for more of your questions!)

Civility is required at all times; rudeness will not be tolerated.

Treat others with the same respect you’d want them to treat you. We’re all here to learn together. Be tolerant of others who may not know everything you know. Bring your sense of humor.

You can ask questions, answer, and suggest edits as an anonymous user, much like Wikipedia. There are some things you won’t be able to do on the site without registering, such as vote. But it’s easy to register.

If you have a registered account on any other sites in our network with at least 200 reputation, you will receive a +100 point reputation bonus when you first log in.

Once logged in, you can gain other key privileges through reputation, most crucially the right to vote.

Reputation is a rough measurement of how much the community trusts you; it is earned by convincing your peers that you know what you’re talking about. Basic use of the site, including asking questions, answering, and suggesting edits, does not require any reputation at all. But the more reputation you earn, the more privileges you gain.

 answer is voted up +10 question is voted up +5 answer is accepted +15 (+2 to acceptor) question is voted down -2 answer is voted down -2 (-1 to voter)

A maximum of 40 votes can be cast per user per day, however, to reach the maximum you must vote on at least 10 questions. You can earn a maximum of 200 reputation per day. Please note that votes for posts marked “community wiki” do not generate any reputation, while accepted answers and bounty awards are not subject to the daily reputation limit.

The other way to gain reputation is by suggesting edits to existing posts as a new registered user. Each edit will be peer reviewed, and if it is accepted, you will earn +2 reputation. You can only earn a maximum of +1000 total reputation through suggested edits, however.

Amass enough reputation points and you will be granted additional privileges:

If you are an experienced Stack Exchange network user with 200 or more reputation on at least one site, you will receive a starting +100 reputation bonus to get you past basic new user restrictions. This will happen automatically on all current Stack Exchange sites where you have an account, and on any other Stack Exchange sites at the time you log in.

At the high end of this reputation spectrum there is little difference between users with high reputation and moderators. That is very much intentional. We don’t run this site. The community does.

If, despite your best efforts, you feel questions aren’t getting good answers, you can help by offering a bounty.

Slice off anywhere from +50 to +500 of your own hard-earned reputation and attach it to any question as a bounty.

The bountied question will appear with a special indicator in all question lists, and it will also be visible on the home page Featured tab for 7 days.

Click the bounty award icon next to each answer to permanently award your bounty to the answerer.

There are a few other rules around bounties:

• Questions must be at least 2 days old to be eligible for a bounty. There can only be 1 active bounty per question at any given time.
• Users must have at least 75 reputation to offer a bounty, and may only have a maximum of 3 active bounties at any given time.
• The bounty period lasts 7 days. Bounties must have a minimum duration of at least 1 day. After the bounty ends, there is a grace period of 24 hours to manually award the bounty.
• If you do not award your bounty within 7 days (plus the grace period), the highest voted answer created after the bounty started with at least 2 upvotes will be awarded half the bounty amount. If there's no answer meeting that criteria, the bounty is not awarded to anyone.
• If the bounty was started by the question owner, and the question owner accepts an answer during the bounty period, and the bounty expires without an explicit award – we assume the bounty owner liked the answer they accepted and award it the full bounty amount at the time of bounty expiration.

In any case, you will always give up the amount of reputation specified in the bounty, so if you start a bounty, be sure to follow up and award your bounty to the best answer!

As an additional bonus, bounty awards are immune to the daily reputation cap and community wiki mode.

Questions that are not a good fit for this site may be voted closed by experienced community members. Closed questions cannot be answered, but are eligible for improvement (and eventual re-opening) through editing, voting, and commenting. See How to Ask for guidance on editing your question to improve it.

Common reasons a question may be closed include:

• exact duplicate
This question covers exactly the same content as earlier questions on this topic; its answers may be merged with another identical question.

• off topic
Questions on Physics Stack Exchange are expected to relate to physics within the scope defined in the FAQ. Consider editing the question or leaving comments for improvement if you believe the question can be reworded to fit within the scope. Read more about closed questions here.

• not constructive
As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or specific expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, see the FAQ for guidance.

• not a real question
It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, see the FAQ.

• too localized
This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, see the FAQ.

Users with 3000 reputation can cast up to 24 close votes per day. When a question reaches 5 close votes, it is marked as closed, and will no longer accept answers. Closed questions may be opened by casting reopen votes in the same manner. However, you may only vote to close or reopen a question once.

Questions that are extremely off topic, or of very low quality, may be removed at the discretion of the community and moderators.

Over time, closed questions that are not useful as signpoints to other questions may also be removed, as well as questions which have no significant activity over a very long period after being asked. For additional guidance, see How to Ask.

Answers that do not fundamentally answer the question may be removed. This includes answers that are …

• commentary on the question or other answers
• “thanks!” or “me too!” responses
• exact duplicates of other answers
• barely more than a link to an external site
• not even a partial answer to the actual question

If you wish to improve an existing answer, click edit. For additional guidance, see How to Answer.

Each post and comment has a small flag link. Click the flag link to let us know about problems and we'll follow up.

We actively moderate our community, but we need your help to do so. Anything that is getting consistently flagged by our community members will be investigated and followed up on. And of course you can always email us directly if you feel the matter is urgent.

Most importantly, don't feed the trolls! Replying to abusive, off-topic, or inappropriate content only encourages it – whereas flagging allows removal without providing undue attention.

Please don’t use signatures or taglines in your posts, or they will be removed.

Your user page belongs to you — fill it with information about your interests, links to stuff you’ve worked on, or whatever else you like!

All contributions are licensed under Creative Commons and this site is collaboratively edited, like Wikipedia. If you see something that needs improvement, click edit and help us make it so!

All edits are tracked in a public revision history. To view revisions, click the edit date on the post.

If you are not comfortable with the idea of your contributions being collaboratively edited by other trusted users, this may not be the site for you.

Be careful, because the community frowns on overt self-promotion and tends to vote it down and flag it as spam. Post good, relevant answers, and if some (but not all) happen to be about your product or website, so be it. However, you must disclose your affiliation in your answers.

If a huge percentage of your posts include a mention of your product or website, you're probably here for the wrong reasons. Our advertising rates are quite reasonable; contact our ad sales team for details. We also offer free community promotion ads for open source projects and non-profit organizations.

Enter search terms in the search box that appears at the upper right of every page, and press Enter.

If you need to refine and narrow your search …

• To search within specific tag(s), enter them in square brackets: [maintenance] seat.
• To find a specific phrase, enter it in quotes: "flat tire".
• To ensure that specific words appear in the results, use plus: +frame +steel.
• To search just your posts: user:me training.

For the complete list of advanced search options, with examples, visit the search page.

If you’re looking for excruciating detail, our meta-discussion site hosts a section of constantly evolving frequently asked questions that document everything about the site. Or, maybe you’d just like to learn a little more about us?