The Laws of Mechanics, which we stated and discussed in Chapter 8, posit the existence of inertial systems, namely those systems where isolated points stay in their state of rest or of linear uniform motion. Newton’s law (8.1) establishes how a particle moves (in an inertial system) under the action of forces. These two principles alone, however, would not be capable of explaining the motion we observe in our day-to-day reality, given that no frame of reference accessible to us behaves as inertial. A frame of reference comoving with Earth is affected by the planet’s rotation; a more precise system comoving with the Sun would still be impacted by the latter’s motion together with the solar system’s.