Volumes – The Disk Method Textbook 7.2 Overview on Page 456
Revolving a Function We have a function f(x) on the interval [a, b] What happens when we revolve that segment of curve about the x axis? What kind of functions generated these solids of revolution? f(x) a b
Disks We can use integrals to determine the volume of these solids f(x) We can use integrals to determine the volume of these solids Here we have a disk which is a slice of the solid What is the radius? What is the thickness? What then, is its volume? dx
Disks To find the volume of the whole solid we sum the volumes of the disks Shown as a definite integral f(x) a b
Try It Out! Try the function y = x3 on the interval 0 < x < 2 rotated about x-axis
Revolve About Line Not a Coordinate Axis Consider the function y = 2x2 and the boundary lines y = 0, x = 2 Revolve this region about the line x = 2 We need an expression for the radius in terms of y
Washers Consider the area between two functions rotated about the axis Now we have a hollow solid We will sum the volumes of washers As an integral f(x) g(x) a b
What will be the limits of integration? Application Given two functions y = x2, and y = x3 Revolve region between about x-axis What will be the limits of integration?
Revolving About y-Axis Also possible to revolve a function about the y-axis Make a disk or a washer to be horizontal Consider revolving a parabola about the y-axis How to represent the radius? What is the thickness of the disk?
Revolving About y-Axis Must consider curve as x = f(y) Radius = f(y) Slice is dy thick Volume of the solid rotated about y-axis
Flat Washer Determine the volume of the solid generated by the region between y = x2 and y = 4x, revolved about the y-axis Radius of inner circle? f(y) = y/4 Radius of outer circle? Limits? 0 < y < 16