It's not a theorem, but that's because it's not proven, or can't be proven really. Unless you're referring to the fact that those three definitions coincide, th

It's not a theorem, but that's because it's not proven, or can't be proven really. Unless you're referring to the fact that those three definitions coincide, that is a theorem. The statement F=ma is either a definition, axiom or theorem depending on what context you're working in. There are more fundamental (empirically tested) axioms that have F=ma as a consequence. Unless you introduce special relativity, then it becomes false.