Velocity is relative because when describing something's velocity you have to say relative to what. i.e. "A particle is traveling at 5m/s" doesn't make sense. "A particle is traveling at 5m/s relative to me" does make sense. Does "A particle is accelerating at 5m/s²" make sense? I have seen this argued before and people sometimes say "Acceleration is not relative because you can feel yourself accelerating" (And this means that you can tell you are accelerating a certain amount, not relative to anything). I disagree with that reasoning because you are feeling yourself being squished, you don't feel yourself accelerating. If your whole body is accelerating equally (or we are just talking about particles accelerating, not bodies) then you cannot measure the acceleration of yourself. So if there are two particles, one is accelerating at 5m/s² directly away from the other. Is it valid to say that one is accelerating and the other one isn't?