Homework Help: Help with washers method about a line

1. Nov 18, 2008

Toanizzle

help with washers method about a line :(

1. The problem statement, all variables and given/known data
We are given two curves y=sin(x) and y=0. Use washers to revolve this around the line y=-2 from 0 to pi.

2. Relevant equations

3. The attempt at a solution
I kno that the equation is 2pi Int( height x radius x thickness), but i have absolutely no idea what to plug in and how to figure out where these things come from :(
I kno how to do it about the x axis, by using pi Int(R(x)-r(x))dx but how do i do it about the line y=-2?

Last edited: Nov 18, 2008
2. Nov 18, 2008

Toanizzle

Re: help with washers method about a line :(

im sorry if i havent had enough of an attempt but im completely lost right no :(

3. Nov 19, 2008

HallsofIvy

Re: help with washers method about a line :(

It would be a good idea to start by stating the problem correctly! "Use washers to revolve this around the line y= 2 from 0 to pi" makes no sense. You don't need "washers" or "disks" or "shells" to revolve anything. You need those to find the volume or the surface area. Was the problem "find the volume of the solid created by revolving around the line y=-2" or "find the surface area of the solid created by revolving around the line y=-2"?

One reason I say that is because "use washers" imply you are finding the volume but the formula you say you "know" is for surface area. We can't help you until you tell us what the problem really is.

4. Nov 19, 2008

Staff: Mentor

Re: help with washers method about a line :(

Have you sketched a graph of the region to be revolved? After you do that, draw a sketch of the cross section in the x-y plane of the solid of revolution. A typical volume element will look like a washer (a disk with a hole in it). Find an expression that gives the volume of your typical volume element, and that will be essentially your integrand.