v' = v or, in general, vm = vn
To precede possible misunderstanding , the meaning of the speed v or v' is the following:
When a body is moving at a speed v in the unprimed frame, the observer from
the primed frame can perform the measurement of this speed (not the resultant speed) by the instruments breathing with the
In both cases the measurement shows the same magnitude of the speed, now designated as
If a body was moving at a speed v' in the primed frame, and the observer from the unprimed frame
similar measurement of the speed v, he would find out, that
v' = v as well.
The speed in a frame M, measured by the instruments not breathing with space and time and calibrated for the time and length
units of the N frame, may be expressed :
and the con-spacetime speed calculated from con-spacetime length and time is defined by equation (5c).