total torsion of closed curve on sphere

**henderson7878** · May 2nd 2013, 02:56 PM

The total torsion of a unit speed curve γ : [a, b] → R3 is defined to be. ∫ τ(s) ds (bounds a to b), where τ(s) is the torsion of γ at the point γ(s). If γ is a closed curve on the unit sphere S2, show that γ has zero total torsion. I have a couple hints to solve this but I can't figure it out, the hints are use k(T)=K_γ cos θ where T=γ' for a unit speed curve γ, and differentiate cosθ=N · U

**xxp9** · May 3rd 2013, 07:12 AM

For the curve r=r(t), its tangent, normal, and binormal unit vectors, often called T, N, and B, satisfies the Frenet–Serret formulas. So we have:
$\frac{dN}{ds}=-\kappa T+\tau B$ .
Let n be the unit normal vector of the sphere, and denote $\theta$ as the angle between n and N such that $\cos\theta=\langle N, n \rangle$
Differentiate the above equation against s the arc length parameter we get:
$-\sin\theta \frac{d\theta}{ds}=\langle \frac{dN}{ds}, n \rangle + \langle N, \frac{dn}{ds} \rangle$
For spheres we have n=r the position vector, so $\frac{dn}{ds}=\frac{dr}{ds}=T$
Plugin this one and the above Frenet formula we get
$-\sin\theta \frac{d\theta}{ds}=\langle -\kappa T+\tau B, n \rangle + \langle N, T \rangle = \tau \langle B, n \rangle$
Since B, N, n are all orthogonal to T, their three are in the same plane. And since the angle between N and n is $\theta$ ,
and B, N are orthogonal, the angle between B and n is then $\theta-\frac{\pi}{2}$ .
So we have $\langle B, n \rangle=\sin\theta$ . Plugin to the above equation we got:
$\tau=-\frac{d\theta}{ds}$
Integrate it we get $\int \tau ds = -\int d\theta = 0$

**math410** · May 5th 2013, 11:51 AM

why does n=r?

**xxp9** · May 5th 2013, 12:31 PM

You said it is a unit sphere. so n=r.

**hughc1992** · May 11th 2013, 03:20 PM

Originally Posted by xxp9

You said it is a unit sphere. so n=r.

Can you explain more about n=r? i still don't get this part. Thanks

**xxp9** · May 12th 2013, 06:47 AM

for the unit sphere S, for any point p on it, the unit normal vector n at p has the same direction and length as p. that is, n=p.

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