Inertia

Inertia is the tendency of a body to maintain its movement indefinitely. This concept was precisely formulated in the "principle of inertia" or Newton's first law: a body subject to no force (or to a system of forces of which the resultant is zero) remains immobile or in uniform rectilinear movement. The principle is only valid for observers in a Galilean reference frame, i.e. at rest, or with a uniform rectilinear movement in relation to the "fixed stars".