Newtons' first law leads to inertial frames. Actually listening to student's questions, requires honest answers here. I put forward limited and pseudo inertia frame. The Newton two is given a "d and t" and we declare work, kinetic energy, impulse and momentum http://lessonpeer.com/drhillteach/What%20can%20I%20do%20with%20Newton%20One%20and%20Two.mp3