# Math Help - Surface rotation

1. ## Surface rotation

A surface S is obtained by rotation, around the axis OZ, the arc of parabola $\alpha : z = \frac{x^2}{2}, y = 0$ with $0 \leq x \leq 1$. Determine the area of the surface obtained

2. How do I calculate surface rotation? Must use the matrix?

3. Originally Posted by Apprentice123
How do I calculate surface rotation? Must use the matrix?

It's a standard formula in Calc II $
SA = 2 \pi \int_a^b y \sqrt{1+y'^2}\,dx
$

4. Originally Posted by Danny
It's a standard formula in Calc II $
SA = 2 \pi \int_a^b y \sqrt{1+y'^2}\,dx
$
I do not know this formula.
Could you explain better?

5. Originally Posted by Apprentice123
I do not know this formula.

Could you explain better?
Are you familar with the formula's for volumes of revolution?

I need a starting place for our discussion.

6. Or you can recognize that the rotation will produce an elliptical paraboloid $z=\frac{x^2+y^2}{2}$ which can be parametrized as $\sigma(u,v)=\left(u,v, \frac{u^2+v^2}{2}\right)$

A switch to polar coordinates will then basically produce the same integral as Danny showed you.

$\iint_D \left| \vec {\sigma'_u}\times\vec {\sigma'_v}\right|dudv=\{\text{Polar coordinates}\}=\int \int_0^{2\pi}\sqrt{r^2+1} \cdot rd\varphi dr=$ $2\pi\int r\sqrt{r^2+1}dr$

7. Originally Posted by Spec
Or you can recognize that the rotation will produce an elliptical paraboloid $z=\frac{x^2+y^2}{2}$ which can be parametrized as $\sigma(u,v)=\left(u,v, \frac{u^2+v^2}{2}\right)$

A switch to polar coordinates will then basically produce the same integral as Danny showed you.

$\iint_D \left| \vec {\sigma'_u}\times\vec {\sigma'_v}\right|dudv=\{\text{Polar coordinates}\}=\int \int_0^{2\pi}\sqrt{r^2+1} \cdot rd\varphi dr=$ $2\pi\int r\sqrt{r^2+1}dr$
Ok. Thanks

8. Originally Posted by Spec
Or you can recognize that the rotation will produce an elliptical paraboloid $z=\frac{x^2+y^2}{2}$ which can be parametrized as $\sigma(u,v)=\left(u,v, \frac{u^2+v^2}{2}\right)$

A switch to polar coordinates will then basically produce the same integral as Danny showed you.

$\iint_D \left| \vec {\sigma'_u}\times\vec {\sigma'_v}\right|dudv=\{\text{Polar coordinates}\}=\int \int_0^{2\pi}\sqrt{r^2+1} \cdot rd\varphi dr=$ $2\pi\int r\sqrt{r^2+1}dr$
as you know that rotation will produce an elliptical paraboloid $z=\frac{x^2+y^2}{2}$