Reference: Einstein’s 1920 Book
Section IV (Part 1)
The Galileian System of Co-ordinates
Please see Section IV at the link above.
.
Summary
The Law of Inertia basically means that in the absence of external forces, a body moves without acceleration at an inherent constant velocity in a straight line.
If we use a system of co-ordinates which is rigidly attached to the earth, then, relative to this system, every fixed star describes a circle of immense radius in the course of an astronomical day. But this is opposed to the statement of the law of inertia, since the visible fixed stars have no motion or acceleration. We, therefore, conclude that the earth is actually rotating around an axis. Thus, the law of inertia also indicates the reference-bodies, or systems of co-ordinates, permissible in mechanics.
The “Galilean system of co-ordinates” is a system of co-ordinates of which the state of motion is such that the law of inertia holds relative to it. The laws of the mechanics of Galilei-Newton can be regarded as valid only for a Galilean system of co-ordinates.
.
Comments
The law of inertia is very specific about zero acceleration and the uniform velocity of the body in a straight line. However, it omits to mention anything about the magnitude of that uniform velocity.
The magnitude of the uniform velocity may depend on the mass of the body, because if the mass of the body is infinite, it cannot have inherent velocity.
.
