Register to reply

3-D cylindrical equation graphing

by NBAJam100
Tags: cylindrical, equation, graphing
Share this thread:
NBAJam100
#1
Sep23-08, 12:38 AM
P: 159
1. The problem statement, all variables and given/known data

Represent the surface in space, identify the surface-

r=2cos(theta)



2. Relevant equations

Uhh....



3. The attempt at a solution

My main thing here is this... how does r tell me anything about the graph of the function in a 3-D plane? i see that r=2cos(theta) but what does it tell me?? i know that 2cos(theta) = cos^2(t)-sin^2(t), so r= that... now what>??? im honestly stumped here, thats really as far as i can go with what i know.
Phys.Org News Partner Science news on Phys.org
FIXD tells car drivers via smartphone what is wrong
Team pioneers strategy for creating new materials
Team defines new biodiversity metric
tiny-tim
#2
Sep23-08, 07:12 AM
Sci Advisor
HW Helper
Thanks
tiny-tim's Avatar
P: 26,148
Quote Quote by NBAJam100 View Post
Represent the surface in space, identify the surface-

r=2cos(theta)
Ö
My main thing here is this... how does r tell me anything about the graph of the function in a 3-D plane? i see that r=2cos(theta) but what does it tell me?? i know that 2cos(theta) = cos^2(t)-sin^2(t), so r= that... now what>??? im honestly stumped here, thats really as far as i can go with what i know.
Hi NBAJam100!

(have a theta: θ and a squared: ≤ )

Can you check the equation?

r can't be negative, and cosθ can be negative, so how can it be r = 2cosθ?

(also, it isn't 2cosθ = cos≤θ-sin≤θ, it's cos2θ)
HallsofIvy
#3
Sep23-08, 07:34 AM
Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 39,552
First, forget "2cos(theta)= cos^2(t)- sin^2(t)". It isn't true. You may be thinking about "cos(2 theta)= cos^2(theta)- sin^2(theta)" but that doesn't help here.

What does r= 2cos(theta) tell you? It tells you the r coordinate: the distance from the point to the z-axis, for each angle theta, of course.

Because this is in cylindrical coordinates, the first thing you should do is think about exactly what cylindrical coordinates are: there are 3 coordinates, of course, r, theta, and z. z measures the height above the xy-plane just as in Cartesian coordinates and r and theta are just the usual polar coordinates except that they apply to each plane parallel to the xy-plane. In particular, do you notice that there is no "z" in the formula? That tells you that the surface looks exactly the same for all different z!

You should also think about the equations connecting cylindrical and Cartesian coordinates: x= r cos(theta), y= r sin(theta), z= z or (inverting) r= [itex]\sqrt{x^2+ y^2}, theta= arctan(y/x), z= z. Your equation is r= 2 cos(theta). What do you get if you multiply both sides of the equation by r?


Register to reply

Related Discussions
Laplace equation, cylindrical 2D Calculus & Beyond Homework 6
Laplace's Equation in Cylindrical Coordinates Advanced Physics Homework 4
Cylindrical general differential equation of conduction Advanced Physics Homework 1
Help graphing the equation of an ellipse. Precalculus Mathematics Homework 16
Need Equation/graphing help Differential Equations 4