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hope you are well,
So , in my physics class we use graphical method to determine the radius of curvature and as we know this method isn't very accurate, so when I watch a YouTube video I find this relation:
So is this relation always true? Because I was able to find it before watching the video, but with the condition of $ay=0$.
$\begingroup$I would consider this to be the definition of the radius of curvature of a curve $\vec{r}(t)$. So it must always be true by definition.$\endgroup$