Introduction to Electrodynamics

Classical mechanics obeys the principle of relativity: the same laws apply in any inertial reference frame. By “inertial” I mean that the system is at rest or moving with constant velocity.1 Imagine, for example, that you have loaded a billiard table onto a railroad car, and the train is going at constant speed down a smooth straight track. The game will proceed exactly the same as it would if the train were parked in the station; you don’t have to “correct” your shots for the fact that the train is moving – indeed, if you pulled all the curtains, you would have no way of knowing whether the train was moving or not.