# Frenet-Serret Formulas

1. Sep 5, 2013

### Philosophaie

I want to substitute ds by dt in the Frenet-Serret Formulas where κ is the curvature and is the torsion:
Tangential:$$\frac{d\vec{T}}{ds} = κ*\vec{N}$$
Normal:$$\frac{d\vec{N}}{ds} = -κ*\vec{T}+τ*\vec{B}$$
Binormal:$$\frac{d\vec{B}}{ds} =- τ*\vec{N}$$
I want to substitute $$\frac{d\vec{T}}{ds} → \frac{d}{dt} T(t)$$ N(t), B(t) and solve for κ and τ.

Last edited: Sep 5, 2013
2. Sep 7, 2013

### vanhees71

Just use the chain rule and the fact that
$$\dot{s}=\frac{\mathrm{d} s}{\mathrm{d} t}=\left | \frac{\mathrm{d} \vec{r}}{\mathrm{d} t} \right|=\left |\dot{\vec{r}} \right|,$$
where the function $\vec{r}(t)$[/itex] is the parametrization of the curve. Then you have
$$\vec{T}=\frac{\dot{\vec{r}}}{\dot{s}}$$
etc.