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 Jun 2 asked How are these two Riemann tensor equations equivalent? May 15 comment Why two different Lagrangians to derive geodesic equations? I couldn't figure it out so I did ask a separate question, here physics.stackexchange.com/questions/183970/…. It was answered very nicely by Horus. The penny has now dropped. Thanks. May 14 comment Carroll's derivation of the geodesic equations @tok3rat0r - hasn't Horus multiplied through twice by $\frac{d\lambda}{d\tau}\frac{d\tau}{d\lambda}$? That gives the $\frac{d\lambda}{d\tau}\frac{d\lambda}{d\tau}d\lambda$ term at the end, which cancels to give $d\tau$. May 14 accepted Carroll's derivation of the geodesic equations May 14 comment Carroll's derivation of the geodesic equations Well, I think I get it now. Very well explained. Thanks. May 14 comment Carroll's derivation of the geodesic equations Although my question has been flagged as being asked before and already having an answer, it wasn't answered at the sub-graduate level I could understand. Horus's answer, on the other hand, I could follow, and many thanks to him for persevering. May 14 comment Carroll's derivation of the geodesic equations Sorry, I realise I'm missing the blindingly obvious here (and being close voted to oblivion), but I can't still can't see how all those denominator $d\lambda$s become $d\tau$s. Do I substitute $d\lambda=\frac{d\lambda}{d\tau}d\tau$ for every single $d\lambda$? May 14 asked Carroll's derivation of the geodesic equations May 14 accepted Why two different Lagrangians to derive geodesic equations? May 13 comment Why two different Lagrangians to derive geodesic equations? Any chance of a hint as to what that chain rule looks like? I cannot see how he substitutes (3.52) into (3.51) to get the equation at the bottom of page 69. Thank you for your patience. May 13 comment Why two different Lagrangians to derive geodesic equations? Thanks, but I just cannot see how he gets $d\tau$ as the denominator after making the affine parameter substitution (Equation 3.52). May 13 comment Why two different Lagrangians to derive geodesic equations? Thanks. I will try to derive the non-affine geodesic equations you give. In the meantime do you have a link to this derivation? I'm still puzzled as to how Moore starts with the square root Lagrangian and ends up with the affine geodesic equations. May 13 comment Why two different Lagrangians to derive geodesic equations? @Qmechanic - thanks. How come Valter Moretti's derivation here (which is over my head) starts with the square root Lagrangian and ends with (what I assume is) the affinely parametrized geodesic equations, ie my first equation? May 12 asked Why two different Lagrangians to derive geodesic equations? May 10 comment Differentiating the Lagrangian to find geodesic equations? Yes, the Kronecker delta renames the index. I see it now. Thanks. May 10 accepted Differentiating the Lagrangian to find geodesic equations? May 10 asked Differentiating the Lagrangian to find geodesic equations? Mar 4 accepted How to tell if a star is in a galaxy? Mar 4 asked How to tell if a star is in a galaxy? Dec 15 awarded Popular Question