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For the origin of K′ we have permanently x′ = 0, and hence

according to the first of the equations (5)

image041

If we call v the velocity with which the origin of K′ is moving

relative to K, we then have

image042

The same value v can be obtained from equations (5), if we

calculate the velocity of another point of K′ relative to K, or

the velocity (directed towards the negative x-axis) of a point of

K with respect to K′. In short, we can designate v as the

relative velocity of the two systems.