In college physics 1, I think I'm confusing myself on reference frames, and would like for one of you significantly smarter persons to let me know if I'm on the right path with understanding it, and if not, could point me in the right direction :) We're just beginning inertia and momentum in both of my classes (physics 1 and mechanics), and the subject of reference frames are starting to become more frequently talked about. In the book (Physics for scientists & engineers, Giancoli, 4th edition), it states that Newtons first 2 laws are only relevant in the inertial reference frames and not in noninertial reference frames, and gives an example of a cup sitting on a dashboard is outside of your inertial reference frame from the cars. What does that mean? Does that mean if we were to calculate the sliding of the cup, it would require its own set of variables outside of the scope of the cars, and because from our perspective, it's just sitting there, but from the cups perspective, it's moving with a constant velocity? Wouldn't that be centripetal force?