We have seen that motion of the body has no meaning unless it is described with respect to some well-defined co-ordinate system. The co-ordinate system is known as the “frame of reference”. Thus a frame of reference is a coordinate system relative to which the position and motion of the body is described. The reference frame may be at rest or under motion. Its choice is totally at our own convenience. Suppose we want to study about anything from Earth. Thus we are considering frame of reference as the Earth. However, Earth is not a stationary object. Similarly, a passenger sitting in a train observes outside trees moving backwards, which is not the fact. Actually in the reference system attached to the earth the train is moving forward.
There are two types of Frames of Reference :
8.2.1 Inertial (unaccelerated)
This obeys Newton’s laws of inertia and other laws of Newtonian mechanics. In this frame, a body not acted upon by external force, is at rest, or moves with a constant velocity. Actually for
different types of frames of references the laws of mechanics becomes different. We can approximate a reference frame which moves with constant velocity relative to the distant stars as a best approximation of inertial frame.
8.2.2 Non-Inertial (accelerated)
This doesn’t obey Newton’s laws of inertia and other laws of Newtonian mechanics. In this frame, a body not acted upon by external force, is accelerated. A coordinate system fixed in the Earth is a non-inertial frame if the object motion is observed from the outside of the Earth as it itself rotates about its axis and also the sun.
Ether: Newton’s concept of stationary frame of reference gave the idea of the ‘ether’ frame in 19th century. As we measure the speed of any moving object with respect to any stationary object, the velocity of light also requires a stationary frame for its velocity measurement. Hence it seemed a fairly logical postulate to think for the ether medium having unusual properties like zero density and 100% transmittivity. These properties are essential for its indefectibility . It was assumed to fill all the space by ether and called as a universal relative to which any motion can be defined.