stackexchange.com is a great site to ask questions and receive
professional answers, but it usually takes time to get an answer and
questions are sometimes closed unnecessarily.
Here are some other options:
If your question is closed on physics.stackexchange.com (which is
plagued by “overly aggressive moderators”), try posting on:
astronomy.stackexchange.com if your question is about astronomy.
space.stackexchange.com if your question is about space
exploration, rocket launches, or anything similar.
earthscience.stackexchange.com if your question relates to Earth
gis.stackexchange.com if your question is about GIS or geography.
mathematica.stackexchange.com if you are working physics
problems with Mathematica(tm) software.
scifi.stackexchange.com if you’re wondering how physics works in
an existing science fiction world, or
worldbuilding.stackexchange.com if you’re trying to create a world
and need help with your fictional physics.
http://meta.physics.stackexchange.com/questions/391 also lists
several possibly helpful sites.
You can also try visiting the Physics StackExchange chat room:
http://chat.stackexchange.com/rooms/71/the-h-bar where the atmosphere
is generally friendlier. In many cases, they will help you with
homework, even if your problem was closed on the main site.
If you need help more urgently, or your question was closed on a
different StackExchange site, you might try:
openstudy.com: near-real-time help, somewhat
wolframalpha.com: if you just need to check your answers, this
can be helpful, but it’s been having a few serious problems later,
so proceed with caution.
quora.com: if your question is more philosophical or
discussion-based (instead of looking for a specific answer), Quora
might be the place to post.
If you would like any other sites listed here, please let me know. The
listings below are from individual users, and I have not vetted them: