How can (or should) I reference basic physics equations?
Justifications are for claims you make that need to be justified. Everyone reading your paper will know about Newton's second law. Everyone reading one of my papers (in quantum theory) will know Schrodinger's equation. I don't think people generally cite them, because everyone will need to know them to understand anything in the paper even a little. 

On the other hand, I don't know what "Coulomb's friction model" is off the top of my head. If you introduce this, you should cite it. In general, if it's something that's well-established (but not necessarily well-known) I often see people referencing a textbook. This is quite common for things like mathematical theories. If it's less well-established or more particular, find a paper or review article.

My main recommendation, however, is to learn from your surroundings. If you're writing a paper, you must be citing a lot of previous work. What do they say, and what do they cite? That can tell you what the expected norms are in your field.