Shell integration


Shell integration is a method for calculating the volume of a solid of revolution, when integrating along an axis perpendicular to the axis of revolution. This is in contrast to disc integration which integrates along the axis parallel to the axis of revolution.

Definition

The shell method goes as follows: Consider a volume in three dimensions obtained by rotating a cross-section in the -plane around the -axis. Suppose the cross-section is defined by the graph of the positive function on the interval. Then the formula for the volume will be:
If the function is of the coordinate and the axis of rotation is the -axis then the formula becomes:
If the function is rotating around the line or, then the formulas become:
and
The formula is derived by computing the double integral in polar coordinates.

Example

Consider the volume, depicted below, whose cross section on the interval is defined by:
In the case of disk integration we would need to solve for given. Because the volume is hollow in the middle we will find two functions, one that defines the inner solid and one that defines the outer solid. After integrating these two functions with the disk method we subtract them to yield the desired volume.
With the shell method all we need is the following formula:
By expanding the polynomial the integral becomes very simple. In the end we find the volume is cubic units.