I usually hear people talk about Newton's 2nd Law like it's a, well, law, as in it relates several defined concepts. I had always thought of it as the definition of force. I'm pretty sure I'm wrong in this respect, but I can't figure out how. If mass times acceleration isn't the definition of force, it must have another definition, and Newton's 2nd Law relates these two concepts. Is it incorrect to think that every physical value that doesn't have base units must have a definition? If not, why not? And if so, what is the definition of force?
I asked my professor this question, and he said that force is defined to be something that changes the velocity of an object. Given that acceleration is the time derivative of velocity, this sounds like a roundabout way of saying $F=ma.$
If I'm actually right, and Newton's 2nd Law is a definition, why do people treat it (and name it) like a law?
