Twisted minds

Calculus Level 3

Consider the function on three variables defined as F ( x , y , z ) = ( x y , y z , z x ) F(x,y,z) = (xy, yz, zx) . Let C C be the twisted cubic polynomial curve given by r ( t ) = ( t , t 2 , t 3 ) \mathbf{r}(t) = (t,t^2,t^3) as t t moves from 0 to 1. What is the value of N = C F d r ? N = \int_C F \cdot dr ?


The answer is 0.9642857143.

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1 solution

Indronil Ghosh
Feb 18, 2014

Each component of F F is given in terms of the x, y, and z components of r ( t ) \textbf{r}(t) . i.e., the x-component of F F is the product of the x and y components of r ( t ) \textbf{r}(t) , the y-component of F F is the product of the y and z components of r ( t ) \textbf{r}(t) , and the z-component of F F is the product of the z and x components of r ( t ) \textbf{r}(t) . So our line integral is: N = C F d r = C F ( r ( t ) ) r ( t ) d t = C ( ( t t 2 ) , ( t 2 t 3 ) , ( t 3 t ) ) ( 1 , 2 t , 3 t 2 ) d t = C ( t 3 , t 5 , t 4 ) ( 1 , 2 t , 3 t 2 ) d t \begin{aligned} N=\int_C \! F\cdot \mathrm{d}r &= \int_C \! F(\textbf{r}(t))\cdot\textbf{r}'\!(t) \ \mathrm{d}t \\ & =\int_C \! ((t\cdot t^2), (t^2 \cdot t^3), (t^3 \cdot t))\cdot(1, 2t, 3t^2) \ \mathrm{d}t \\ &=\int_C \! (t^3, t^5, t^4)\cdot(1, 2t, 3t^2) \ \mathrm{d}t \end{aligned} Taking the dot product and integrating from 0 to 1: 0 1 ( t 3 + 5 t 6 ) d t = [ 1 4 t 4 + 5 7 t 7 ] 0 1 = 27 28 0.9643 \Rightarrow \int_{0}^{1} \! (t^3+5t^6) \ \mathrm{d}t = \left[\frac{1}{4}t^4+\frac{5}{7}t^7\right]_{0}^{1} = \frac{27}{28} \approx \boxed{0.9643}

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