# vector calculus..cylindrical coordinates

• Sep 10th 2009, 12:23 PM
holly123
vector calculus..cylindrical coordinates
how to you transform cartesian equations into cylindrical coordinates?
for example, write the following cartesian equation in cylindrical coordinates and graph:
x^2 + y^2 + (z-1)^2= 1

this seems like it is really simple, so i'm sorry for posting it, but there aren't any examples in my book so i'm not sure what to do. thanks so much
• Sep 10th 2009, 12:26 PM
Calculus26
let t= theta

x= rcos(t) y = rsin(t) z = z

x^2 + y^2 + (z-1)^2= 1

becomes r^2 +(z-1)^2 = 1
• Sep 10th 2009, 12:28 PM
holly123
• Sep 10th 2009, 12:35 PM
Calculus26
z is the same in cylindrical coord as in rect coord

i.e. the distance above or below the x-y plane or if you will the r-theta plane
• Sep 10th 2009, 12:37 PM
holly123
oh okay that makes sense..but how am i supposed to graph that?
• Sep 10th 2009, 12:43 PM
Calculus26
It is still a sphere centered at (0,0,1)

the idea of the exercise is to recognize the from a sphere takes in cylindrical coordinates

For eg a sphere centered at the origin is r^2 + z^2 = 1

as another eg the saddle z = 1- x^2 + y^2

takes the form z= 1 - r^2cos^2(t) + r^2sin^(t)

z = 1 -r^2cos(2t)

and so on.
• Sep 10th 2009, 12:46 PM
holly123
okay that sorta makes sense to me haha so is the radius 1?

my next problem is z=2(x^2+y^2)..so that would become z=2r^2? is that a sphere too?

sorry for so many questions (Doh)
• Sep 10th 2009, 12:51 PM
Calculus26
You've got it--don't worry about the number of questions

it is always hard when you have to start thinking in terms of a new coordinate system--wait till you get to spherical coords.
• Sep 10th 2009, 12:55 PM
holly123
lol thats the other half of this homework..spherical coords...i almost like them better!

so z=2r^2 has a center of (2,2,0) and i'm not sure about the radius?
• Sep 10th 2009, 12:59 PM
Calculus26
No

z=2(x^2+y^2).. is a parabaloid with vertex (0,0,0)

z =2r^2 -- its still a parabaloid opening up with vertex at (0,0,0)

just written in terms of cylindrical coords

• Sep 10th 2009, 01:00 PM
holly123
thanks so much..its just not clicking for me somehow..i hate 3d!
• Sep 10th 2009, 01:04 PM
Calculus26
the thing to remember is that all the surfaces you learned in rectangular coordinates don't change when you go to cylindrical and spherical coordinates.

The form of the equations change in the new coordinates but you still are describing the same exact surfaces.

You may hate it now but believe me when you get to triple integrals you'll thank your lucky stars for the new coordinate systems

Hang in there