4 - Newtonian relativity

Summary

It has been emphasised in previous chapters that position, and hence also velocity and acceleration, are relative concepts. Thus when stating or defining certain positions some frame of reference is always, at least implicitly, assumed. For example, a child may be told to sit still in a car, even though, together with the car, he may be moving at a great speed. In this case it is obviously implied that the child is required not to move relative to the car. Alternatively, the positions of the pieces in a game of chess are usually stated relative to the chessboard, and are therefore unambiguously defined, even if the board were to be moved from one room to another. In the same way, the items of furniture in a room can be described as being located in fixed positions relative to the floor and walls, in spite of the fact that they have a large velocity due to the rotation of the earth and its orbital velocity about the sun.

Clearly then, positions and dynamical properties such as velocities and accelerations can only be stated or determined relative to an assumed frame of reference. When an individual is required to state a position, he is free to choose an arbitrary frame of reference for convenience. However, when he attempts to measure a dynamical property such as the velocity or acceleration of a particle, in practice he would usually make the measurements relative to himself or to the instruments he is using.