Vector Field / Line Integral (Part 3)

Calculus Level 5

The vector field F \vec{F} has components ( F x , F y , F z ) = ( x y , y z , z x ) (F_x,F_y,F_z) = (x y , y z , z x) . Consider the integration path C C , which is a semicircle starting at P 1 = ( 0 , 0 , 0 ) \vec{P_1} = (0,0,0) and ending at P 2 = ( 1 , 2 , 3 ) \vec{P_2} = (1,2,3) (see "details" section). The diameter of the semicircle is D D , and its radius is R R .

P = P 1 + α u + β v ( R α ) 2 + β 2 = R 2 0 α D \large{\vec{P} = \vec{P_1} + \alpha \, \vec{u} + \beta \, \vec{v} \\ (R - \alpha)^2 + \beta^2 = R^2 \\ 0 \leq \alpha \leq D}

What is the maximum possible value of the line integral of the vector field over the path, given the constraints on the path?

C F d \large{\int_C \vec{F} \cdot \vec{d \ell}}

Details and Assumptions:
1) Vector u \vec{u} is a unit-vector in the direction of ( 1 , 2 , 3 ) (1,2,3)
2) Vector v \vec{v} is a unit-vector perpendicular to u \vec{u} , which remains constant over the path


The answer is 10.95.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

p1 = { 0 , 0 , 0 } \text{p1}=\{0,0,0\}

p2 = { 1 , 2 , 3 } \text{p2}=\{1,2,3\}

center = p1 + p2 2 = { 1 2 , 1 , 3 2 } \text{center}=\frac{\text{p1}+\text{p2}}{2} = \left\{\frac{1}{2},1,\frac{3}{2}\right\}

radius = EuclideanDistance [ p1 , center ] = 7 2 \text{radius}=\text{EuclideanDistance}[\text{p1},\text{center}] =\sqrt{\frac{7}{2}}

The unit vectors that used: ( 1 14 2 7 3 14 13 14 2 91 3 182 0 3 13 2 13 ) \left( \begin{array}{ccc} -\frac{1}{\sqrt{14}} & -\sqrt{\frac{2}{7}} & -\frac{3}{\sqrt{14}} \\ \sqrt{\frac{13}{14}} & -\sqrt{\frac{2}{91}} & -\frac{3}{\sqrt{182}} \\ 0 & -\frac{3}{\sqrt{13}} & \frac{2}{\sqrt{13}} \\ \end{array} \right)

Scale a vector v \vec{v} back to original coordinates: radius v + center ] \text{radius} \vec{v}+\text{center}]

Compute a coordinate from \tau), the path integration variable, and θ \theta , the angle the path is displaced. { 1 2 ( 13 cos ( θ ) cos ( τ ) + sin ( τ ) ) , cos ( θ ) cos ( τ ) 13 3 7 26 sin ( θ ) cos ( τ ) + sin ( τ ) , cos ( τ ) ( 2 14 sin ( θ ) 3 cos ( θ ) ) 2 13 + 3 sin ( τ ) 2 } \left\{\frac{1}{2} \left(\sqrt{13} \cos (\theta ) \cos (\tau )+\sin (\tau )\right),\\-\frac{\cos (\theta ) \cos (\tau )}{\sqrt{13}}-3 \sqrt{\frac{7}{26}} \sin (\theta ) \cos (\tau )+\sin (\tau ),\frac{\cos (\tau ) \left(2 \sqrt{14} \sin (\theta )-3 \cos (\theta )\right)}{2 \sqrt{13}}+\frac{3 \sin (\tau )}{2}\right\}

The integral: 0 π 52 14 sin ( 2 θ ) ( 49 sin ( τ ) + 61 sin ( 2 τ ) ) sin 2 ( τ 2 ) 104 sin 3 ( τ 2 ) cos ( τ 2 ) ( cos ( 2 θ ) ( cos ( τ ) 37 ) + 403 cos ( τ ) 299 ) + 13 13 cos ( θ ) sin 2 ( τ 2 ) ( 38 cos ( τ ) 453 cos ( 2 τ ) + 411 ) + 13 182 sin ( θ ) sin 2 ( τ 2 ) ( 86 cos ( τ ) + 81 cos ( 2 τ ) + 25 ) 13 sin 2 ( τ ) cos ( τ ) ( 943 14 sin ( 3 θ ) + 919 cos ( 3 θ ) ) 5408 d τ = 1 624 ( 50 14 sin ( 2 θ ) + 51 π 182 sin ( θ ) + 224 cos ( 2 θ ) + 294 π 13 cos ( θ ) + 2600 ) \int_0^{\pi } \frac{52 \sqrt{14} \sin (2 \theta ) (49 \sin (\tau )+61 \sin (2 \tau )) \sin ^2\left(\frac{\tau }{2}\right)-104 \sin ^3\left(\frac{\tau }{2}\right) \cos \left(\frac{\tau }{2}\right) (\cos (2 \theta ) (\cos (\tau )-37)+403 \cos (\tau )-299)+\\ 13 \sqrt{13} \cos (\theta ) \sin ^2\left(\frac{\tau }{2}\right) (38 \cos (\tau )-453 \cos (2 \tau )+411)+13 \sqrt{182} \sin (\theta ) \sin ^2\left(\frac{\tau }{2}\right) (-86 \cos (\tau )+81 \cos (2 \tau )+25)-\\ \sqrt{13} \sin ^2(\tau ) \cos (\tau ) \left(943 \sqrt{14} \sin (3 \theta )+919 \cos (3 \theta )\right)}{5408} \, d\tau =\\ \frac{1}{624} \left(50 \sqrt{14} \sin (2 \theta )+51 \pi \sqrt{182} \sin (\theta )+224 \cos (2 \theta )+294 \pi \sqrt{13} \cos (\theta )+2600\right)

Maximize the result over θ \theta and evaluate the maximization result numerically. The maximum is 10.9596177774 10.9596177774 . The angle from u 2 \vec{u_2} is 0.524769623149 0.524769623149 radians.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...