For this site and in general for academic purposes: How can I use copyrighted stuff as material? If you can quote someone why can you not quote their work as well? The teacher in a previous class (not physics) told us not to copy word for word but to recall material in our own words. Does that bypass the problem with plagiarism and copyrighted material being used as an answer? Does that also mean I have to redraw an illustration or find one not copyrighted?
Plagiarism and violations of copyright are two different ideas.
Plagiarism is the use of another persons words or ideas without giving proper credit. It's an academic idea and a violation of academic ethics.
Plagiarism is against the rules, and physics being an academic discipline, the users of this site mostly take those rules very seriously. You can expected plagiarized posts to be deleted. Moreover, you may find that some users don't approve of large-scale copying even when it is properly cited. That disapproval might be expressed in terms of down votes.
You should note that long established physics is understood to be long established and you won't see users citing Newton every time they take force as proportional to acceleration, so there is a degree practicality to the application of citation rules. None-the-less, when there is any doubt you should cite sources.
Certainly if you quote any material from a source directly you should (a) use quotation markup (begin lines with
>), (b) report where you got it with a link and or bibliographic information, and (c) name the actual author.
Copyright violation Is the un-licensesd use of copyrighted material in a way not covered by the doctrine of fair use. This is at a minimum a civil violation of the law (and in some cases can be a criminal violation, though that seems unlikely to apply when we're talking about a Stack Exchange post even though IANAL).
Posts that violate copyright should be deleted or edited to conform, and that can mean finding replacements for copyrighted materials that are in the public domain or covered by a permissive license (google advance search can help with that). In the worst case you might have to construct your own figure.