Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning
ISBN 10: 1285741552
ISBN 13: 978-1-28574-155-0

Chapter 13 - Section 13.3 - Arc Length and Curvature - 13.3 Exercise - Page 870: 66

Answer

The curvature at the point $(0,1,0) $ is: $$ \kappa (0)=\frac{1}{2 \cosh ^{2} (0)}=\frac{1}{2}, $$ The torsion at the point $(0,1,0) $ is: $$ \tau (0)=\frac{-1}{2 \cosh ^{2} t}=\frac{-1}{2}. $$

Work Step by Step

The parametric equations of the curve are: $$ x=\sinh t, \ \ y=\cosh t, \ \ z=t $$ So, the curve is: $$ \mathrm{r}=\left\langle \sinh t, \cosh t, t \right\rangle $$ The curvature formula of the curve is given by: $$\kappa=\left|\frac{d \mathbf{T}}{d s}\right|=\frac{\left|\mathbf{T}^{\prime}(t)\right|}{\left|\mathbf{r}^{\prime}(t)\right|}=\frac{\left|\mathbf{r}^{\prime}(t) \times \mathbf{r}^{\prime \prime}(t)\right|}{\left|\mathbf{r}^{\prime}(t)\right|^{3}} $$ We first compute the ingredients needed $$ \begin{aligned} \mathrm{r}^{\prime} &=\left\langle \cosh t, \sinh t, 1 \right\rangle,\\ \mathrm{r}^{\prime \prime}&=\langle \sinh t,\cosh t, 0 \rangle,\\ \mathrm{r}^{\prime \prime \prime}&=\langle \cosh t ,\sinh t ,0\rangle \end{aligned} $$ and $$ \begin{aligned} \mathbf{r}^{\prime} \times \mathbf{r}^{\prime \prime} &=\left\langle-\cosh t, \sinh t, \cosh ^{2} t-\sinh ^{2} t\right\rangle \\ &=\langle-\cosh t, \sinh t, 1\rangle \end{aligned} $$ The curvature formula of the given curve is: $$ \begin{aligned} \kappa &=\frac{\left|\mathbf{r}^{\prime} \times \mathbf{r}^{\prime \prime}\right|}{\left|\mathbf{r}^{\prime}\right|^{3}} \\ &=\frac{|(-\cosh t, \sinh t, 1\rangle|}{|\langle\cosh t, \sinh t, 1)|^{3}} \\ &=\frac{\sqrt{\cosh ^{2} t+\sinh ^{2} t+1}}{\left(\cosh ^{2} t+\sinh ^{2} t+1\right)^{3 / 2}} \\ &=\frac{1}{\cosh ^{2} t+\sinh ^{2} t+1} \\ &=\frac{1}{2 \cosh ^{2} t}. \end{aligned} $$ The torsion of the given curve, by using the formula in Exercise 63(d), is: $$ \begin{aligned} \tau &=\frac{\left(\mathbf{r}^{\prime} \times \mathbf{r}^{\prime \prime}\right) \cdot \mathbf{r}^{\prime \prime \prime}}{\left|\mathbf{r}^{\prime} \times \mathbf{r}^{\prime \prime}\right|^{2}} \\ &=\frac{\langle-\cosh t, \sinh t, 1\rangle \cdot\langle\cosh t, \sinh t, 0\rangle}{\cosh ^{2} t+\sinh ^{2} t+1} \\ &=\frac{-\cosh ^{2} t+\sinh ^{2} t}{2 \cosh ^{2} t} \\ &=\frac{-1}{2 \cosh ^{2} t} \end{aligned} $$ Thus, the curvature at the point $(0,1,0) $ is: $$ \kappa (0)=\frac{1}{2 \cosh ^{2} (0)}=\frac{1}{2}, $$ The torsion at the point $(0,1,0) $ is: $$ \tau (0)=\frac{-1}{2 \cosh ^{2} t}=\frac{-1}{2}. $$
Update this answer!

You can help us out by revising, improving and updating this answer.

Update this answer

After you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.