
 Inertia and Relativity 
  To answer this objection, Galileo developed the concept of inertia, which he illustrated by “thought experiment” involving the behavior of objects inside a ship which is moving at some constant speed in a straight line. 
  Also, notice that the principle of inertia does not discriminate between different states of uniform motion in straight lines, so it automatically entails a principle of relativity of dynamics, and in fact the two are essentially synonymous. 
  In other words, we define an inertial coordinate system as a system of space and time coordinates in terms of which inertia is homogeneous and isotropic, and then we announce the “laws of motion”, which consist of the assertion that inertia is homogeneous and isotropic with respect to inertial coordinate systems. 
 www.mathpages.com /rr/s103/103.htm (3029 words) 
