# Proving this curve has speed $r$ and torsion $-r^2$

Let $\beta : I \subset \mathbb{R} \rightarrow \mathbb{E}^3 : s \mapsto \beta(s)$ be an arc length parameterized curve defined on the sphere with radius $r$. Define the curve $\alpha$ as follows: $$\alpha (t) = \int_a^t \beta(s) \times \beta' (s) ds.$$ Prove that $\alpha$ has speed $r$ and torsion $-r^2$.

Attempt: By definition the speed is $v = || \alpha' (t) ||$. Now we have that $\alpha' (t) = \beta(t) \times \beta' (t)$. But I'm not sure what to do with this crossproduct. Also, we have the torsion $\tau$ as $$\tau= \frac{ \alpha' \times \alpha'' \cdot \alpha'''}{ || \alpha' \times \alpha'' ||^2 }$$ I computed \begin{align*} \alpha'' &= \beta' (t) \times \beta' (t) + \beta(t) + \beta'' (t) \\ &= \beta(t) \times T'(t) = \kappa v \beta(t) \times N(t) \end{align*} where $\kappa$ is the curvature and $N(t)$ is the unit normal. But I'm not really sure how to continue and how to work out these crossproducts.

• Are you sure the torsion isn't meant to be $r^{-2}$ or maybe $-r^{-2}$? Other variants of this question seem to say so. See for example math.stackexchange.com/questions/3843034/…. Cheers! Oct 17, 2020 at 4:50

From the fact that $\beta$ is on a sphere we know $\|\beta\|$ = r, or $\beta \cdot \beta = r^2$. Differentiating over $s$ you get that $\beta$ and $\beta'$ are orthogonal. The norm of a cross product is the product of norms of factors times the $\sin(angle)$. Therefore $\|\alpha'\| = \|\beta\| \|\beta'\| \sin \frac\pi2 = r\cdot 1\cdot 1 = r$
• I see, thanks. But how did you deduce that $|| \beta' || = 1$ ? Jan 9, 2016 at 19:10
• $\beta$ is parametrized by arc length means that it is unit speed. Jan 9, 2016 at 19:15
• Yes but when you compute $\alpha'$ the parameter $s$ is changed to $t$. I know that $\beta' (s) = 1$, but is this still the case for $\beta' (t)$ or doesn't it really matter? Jan 9, 2016 at 19:17
• it's just the matter of notation, both $s,t \in I$ Jan 9, 2016 at 19:20
Here's a big hint: Try reparametrizing $\alpha$ with respect to arc length. Since you know that the speed of alpha is $r$, you can reparametrize setting $t=rs$. From there it's just a matter of computation.