# Describe the surface in cylindrical coordinates?

Colts

## Homework Statement

The surface is x^2/y*z=10. Put this into cylidrical coordinates. in the form r=f(theta,z)

No clue

No clue

## Answers and Replies

damabo
in your book, you will probably find equations for x, for y in terms of theta and z. don't have my book here with me.

Colts
I don't understand what r=f(theta,z) means and how to write my answer in that form

damabo
apparently, this is the conversion from cartesian coordinates (x,y,z) to cilindrical (rho, phi, z):

$x = \rho \cos \varphi$
$y = \rho \sin \varphi$

$\rho = \sqrt{x^{2}+y^{2}}$ and

$\varphi = \begin{cases} 0 & \mbox{if } x = 0 \mbox{ and } y = 0\\ \arcsin(\frac{y}{\rho}) & \mbox{if } x \geq 0 \\ -\arcsin(\frac{y}{\rho}) + \pi & \mbox{if } x < 0\\ \end{cases}$

Science Advisor
Homework Helper
Gold Member
I don't understand what r=f(theta,z) means and how to write my answer in that form

Substitute ##x=r\cos\theta,\, y=r\sin\theta## in the equation and solve it for ##r##.

damabo
f(theta, z) means that you should equate radius to a function of theta and z. in this case theta is the angle [itex]\phi [/phi] in the equations above. this angle, simply put, is the same as the angle in polar coordinates. the only difference between polar coordinates and cilindrical, is that with cilindrical, you have height (z) as well

Science Advisor
Homework Helper
Gold Member
in this case theta is the angle [itex]\phi [/phi] in the equations above
##\theta## is given by \theta, not \phi.

Colts
Got it. Thanks guys