The first law of motion says that it takes force to accelerate something. The second law of motion says that F=ma. So now my teacher says that the first law is for inertial reference frames, while the second is for non-inertial reference frames. This really annoys me because I don't understand how they are related. I thought that both laws only work in inertial reference frames, or else we'll have to add "fictional" forces" like centrifugal forces, right? Why does the second law apply to non-inertial reference frames?