[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.