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Special K
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In FLP Volume 1 section 17.4, Feynman derives the 4 momentum. He gives the expression for v'(velocity in the moving reference frame) then says to find E' we need to square v', subtract it from one, take the squad root, and take the reciprocal. He does this to get E' is simply mo times the above procedure performed to v'. Why is this procedure correct? I do not understand why those Operations on v' then multiplying by mass give E' necessarily? Unless it just worked out that way?