Curvilinear integral doubt

**Ulysses** · November 13th 2010, 02:58 PM

Hi. I have a doubt with this exercise. I'm not sure about what it asks me to do, when it asks me for the curvilinear integral. The exercise says:

Calculate the next curvilinear integral:

$\displaystyle\int_{C}^{}(x^2-2xy)dx+(y^2-2xy)dy$ , C the arc of parabola $y=x^2$ which connect the point $(-2,4)$ y $(1,1)$
I've made a parametrization for C, thats easy: $\begin{Bmatrix} x=t \\y=t^2\end{matrix}$ $\begin{Bmatrix} x'(t)=1 \\y'(t)=2t\end{matrix}$

And then I've made this integral:
$\displaystyle\int_{-2}^{1}t^2-2t^3+(t^4-2t^3)2t dt$
But now I'm not too sure about this. What I did was:

$\displaystyle\int_{a}^{b}F(\sigma(t))\sigma'(t)dt$

But now I don't know if I should use the module, I did this: $\displaystyle\int_{a}^{b}F(\sigma(t))\sigma'(t)dt$ and I dont know when I should use this: $\displaystyle\int_{a}^{b}F(\sigma(t)) \cdot ||\sigma'(t)||dt$

I mean, both are curvilinear integrals, right?

I think that I understand what both cases means, but I don't know which one I should use when it asks me for the "curvilinear integral". The first case represents the area between the curve and the trajectory, and the second case represents the projection of a vector field over the trajectoriy, i.e. the work in a physical sense, but I know it have other interpretations and uses.

Well, thats all. Bye there, thanks for posting.

**Prove It** · November 13th 2010, 03:24 PM

Originally Posted by Ulysses

Hi. I have a doubt with this exercise. I'm not sure about what it asks me to do, when it asks me for the curvilinear integral. The exercise says:

Calculate the next curvilinear integral:

$\displaystyle\int_{C}^{}(x^2-2xy)dx+(y^2-2xy)dy$ , C the arc of parabola $y=x^2$ which connect the point $(-2,4)$ y $(1,1)$
I've made a parametrization for C, thats easy: $\begin{Bmatrix} x=t \\y=t^2\end{matrix}$ $\begin{Bmatrix} x'(t)=1 \\y'(t)=2t\end{matrix}$

And then I've made this integral:
$\displaystyle\int_{-2}^{1}t^2-2t^3+(t^4-2t^3)2t dt$
But now I'm not too sure about this. What I did was:

$\displaystyle\int_{a}^{b}F(\sigma(t))\sigma'(t)dt$

But now I don't know if I should use the module, I did this: $\displaystyle\int_{a}^{b}F(\sigma(t))\sigma'(t)dt$ and I dont know when I should use this: $\displaystyle\int_{a}^{b}F(\sigma(t)) \cdot ||\sigma'(t)||dt$

I mean, both are curvilinear integrals, right?

I think that I understand what both cases means, but I don't know which one I should use when it asks me for the "curvilinear integral". The first case represents the area between the curve and the trajectory, and the second case represents the projection of a vector field over the trajectoriy, i.e. the work in a physical sense, but I know it have other interpretations and uses.

Well, thats all. Bye there, thanks for posting.

You're correct up to

$\displaystyle \int_{-2}^1{t^2 - 2t^3+(t^4 - 2t^3)2t\,dt}$

$\displaystyle = \int_{-2}^1{t^2 - 2t^3 + 2t^5 - 4t^4\,dt}$ .

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