[burkaman]

It is. It's benefiting from someone else's work without thanking them. This is basic elementary school ethics.

It's equivalent to performing a symphony, but not mentioning the music was written by Beethoven. Not illegal, but wrong, even if you don't explicitly claim it's your own work.

[zaroth]

No, it's not. A symphony can be copyrighted.

It's the same as calculating force equals mass times acceleration, without thanking Newton every time. That's an example (natural law) which is always and immediately public domain, the same as recipes.

[burkaman]

But Beethoven symphonies are not copyrighted, they've been in the public domain for centuries. Is it ok if you use Beethoven?

f=ma is an interesting example. I still think you should mention that it's Newton's Second Law if you're teaching it, and I doubt there's a textbook out there that doesn't. If Newton had discovered this law last year and you wrote a book about it without mentioning him, that would be deeply unethical.