Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 14 - Calculus of Vector-Valued Functions - 14.3 Arc Length and Speed - Exercises - Page 726: 31

Answer

The arc length parametrization: ${{\bf{r}}_1}\left( s \right) = (\cos \left( {{{\left( {\frac{{3s}}{2} + 1} \right)}^{2/3}} - 1} \right),\sin \left( {{{\left( {\frac{{3s}}{2} + 1} \right)}^{2/3}} - 1} \right),$ ${\ \ \ \ \ \ \ \ \ }$ $\frac{2}{3}{\left( {{{\left( {\frac{{3s}}{2} + 1} \right)}^{2/3}} - 1} \right)^{3/2}})$

Work Step by Step

We have ${\bf{r}}\left( t \right) = \left( {\cos t,\sin t,\frac{2}{3}{t^{3/2}}} \right)$. So, the start point $\left( {1,0,0} \right)$ corresponds to $t=0$. The derivative is ${\bf{r}}'\left( t \right) = \left( { - \sin t,\cos t,{t^{1/2}}} \right)$ Evaluate the arc length function: $s\left( t \right) = \mathop \smallint \limits_0^t ||{\bf{r}}'\left( u \right)||{\rm{d}}u$ $s\left( t \right) = \mathop \smallint \limits_0^t \sqrt {{{\sin }^2}u + {{\cos }^2}u + u} {\rm{d}}u = \mathop \smallint \limits_0^t \sqrt {1 + u} {\rm{d}}u$ Write $v=1+u$. So, $dv=du$. The integral becomes $s\left( t \right) = \mathop \smallint \limits_1^{1 + t} {v^{1/2}}{\rm{d}}v = \frac{2}{3}{v^{3/2}}|_1^{1 + t}$ $s\left( t \right) = \frac{2}{3}\left( {{{\left( {1 + t} \right)}^{3/2}} - 1} \right)$ It follows that $\frac{{3s}}{2} + 1 = {\left( {1 + t} \right)^{3/2}}$ ${\left( {\frac{{3s}}{2} + 1} \right)^{2/3}} = 1 + t$ $t = {\left( {\frac{{3s}}{2} + 1} \right)^{2/3}} - 1$ The inverse of $s\left( t \right)$ is $t = {\left( {\frac{{3s}}{2} + 1} \right)^{2/3}} - 1$. Since the start point $\left( {1,0,0} \right)$ corresponds to $t=0$, so at the start point we have $s=0$. Substituting $t$ in ${\bf{r}}\left( t \right) = \left( {\cos t,\sin t,\frac{2}{3}{t^{3/2}}} \right)$ gives the arc length parametrization: ${{\bf{r}}_1}\left( s \right) = (\cos \left( {{{\left( {\frac{{3s}}{2} + 1} \right)}^{2/3}} - 1} \right),\sin \left( {{{\left( {\frac{{3s}}{2} + 1} \right)}^{2/3}} - 1} \right),$ ${\ \ \ \ \ \ }$ $\frac{2}{3}{\left( {{{\left( {\frac{{3s}}{2} + 1} \right)}^{2/3}} - 1} \right)^{3/2}})$
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