3.1. Newton's first law

Newton's first law says: every body maintains a state of rest or a uniform rectilinear motion, until exposure from other bodies causes it to change this state. The first law of Newton shows that the state of rest or uniform rectilinear motion does not require external influences for its maintenance: This manifests a special dynamic property of bodies, called inertness. Accordingly, the first Newton's law is usually called the law of inertia, and the motion of a body free from external influences is called motion by inertia.

Experience shows that Newton's first law is not satisfied in any frame of reference. The reference systems in relation to which the law of inertia is fulfilled are called inertial reference systems. That is, these are such reference systems with respect to which the material point, on which other bodies do not act, either rests or moves uniformly and straightforwardly.