Inertial frame
There exist frames of reference—inertial frames—in which a free particle has constant velocity:
Law 1.1 (Law of Inertia)
There exists inertial frames, in which a free particle has a constant velocity.
Free particles and Newton's 1st law
A free particle, one that experiences no forces, moves with constant velocity in an inertial frame:
The law of inertia is an improved version of Newton’s 1st law. Rather than singling out a natural state of rest that forces maintain or disturb, it asserts uniform motion of free particles in every inertial frame; being at rest is itself just constant velocity in a suitable frame.
Related
Stated in
- Law 1.1 (Law of Inertia)§1 The Structure of the Newtonian Universe