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## Main Question or Discussion Point

I want to know why the 4-velocity vector of a particle is

u'=dx^i/ds where ds=cdt sqrt{1-v^2/c^2}

not dx^i/dt?

also can anyone show me the devide of the 4-acceleration vector,i.e

a' = d^2x'/ds^2?! I dont konw how to write it back in terms of v,c &x^i !!

In addition why the 4-velocity vector is a dimesnsionless quantity? what does it really mean?

thx for help

yukyuk

u'=dx^i/ds where ds=cdt sqrt{1-v^2/c^2}

not dx^i/dt?

also can anyone show me the devide of the 4-acceleration vector,i.e

a' = d^2x'/ds^2?! I dont konw how to write it back in terms of v,c &x^i !!

In addition why the 4-velocity vector is a dimesnsionless quantity? what does it really mean?

thx for help

yukyuk