Einstein's Relativity : Chapter 2

System of Coordinates

Hello, Friends. In the hope that you enjoyed the last lesson, I present you the second lesson. Go out for a while and look up at the stars in the sky. The first thing we observe is that they are far away from us. And this would definitely remind a real physicist or a physics lover the fundamental law of the mechanics of Galilee– Newton, which is also known as the law of inertia, which may be stated as: a body removed sufficiently far from other bodies continues in a state of rest or uniform motion in a straight line. The Law does not talks only of the state of motion or rest of a body but also tells us about the system of coordinates and reference system, which are permissible in the mechanics, which can be used for the mechanical description. Let’s come back to stars I told you to look at. The visible stars are bodies for which this law certainly holds good. Now if we take our reference system rigidly attached to Earth, then, relative to this system of ours every star describes a circular path of a very big radius in a given time period. But… Read the law again. It told us a body removed sufficiently far away from other bodies continues to be in a state of rest or uniform motion. Doesn’t our last observation in which the reference system was the Earth contradicts with the law? Yes, it clearly does. Means this law is not true for a reference system attached to Earth. So by this fact we got to know that Earth itself is not an inertial frame. To keep this law true, we now would have to choose such a system of coordinates in which this law holds true i.e. an inertial frame. Such a system in which this law holds true is called as ‘Galilean system of coordinates’.
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