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Mar7-12, 04:30 AM
Sci Advisor
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tiny-tim's Avatar
P: 26,148
Hi ENDLESSYOU! Welcome to PF!

We have to assume that the observers in S and S' are using units (of distance and time) which make their speeds equal
if S was using metres, and S' was using feet, it wouldn't work!
(btw, this has nothing to do with einstein, it works for galileo also)