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We start off again from quite special cases, which we have

frequently used before. Let us consider a space time domain in

which no gravitational field exists relative to a reference-body

K whose state of motion has been suitably chosen. K is then a

Galileian reference-body as regards the domain considered, and

the results of the special theory of relativity hold relative to

K. Let us suppose the same domain referred to a second body of

reference K′, which is rotating uniformly with respect to K. In

order to fix our ideas, we shall imagine K′ to be in the form of

a plane circular disc, which rotates uniformly in its own plane

about its centre. An observer who is sitting eccentrically on the

disc K′ is sensible of a force which acts outwards in a radial

direction, and which would be interpreted as an effect of inertia

(centrifugal force) by an observer who was at rest with respect

to the original reference-body K.