### 6.3 Volumes by Cylindrical Shells

In 6.2 which Claire so wonderfully covered, we learned how to find volumes using cross-sections of solids perpendicular to the axis of rotation. We did this using the disk and washer methods:

However, the volumes of certain solids cannot easily be found using these two methods. Sometimes it is necessary to use another method: the method of cylindrical shells. Such can be used when the the perpendicular R and r of a washer depend on the same curve. Instead of using cross-sections perpendicular to the axis of rotation, we can use cross-sections parallel to the axis of rotation. Rotating the parallel cross sections about an axis create shells....

In this next picture of a shell, you can see that the thickness of the shell is the difference between radii.

So, the formula for the volume of a cylindrical shell is:

, where r is the average of the radii and is the difference of the radii.

In words, Volume = (circumference)(height)(thickness). This is easier to understand if you imagine the shell cut and rolled out to form a rectangular solid with length , height h, and width .

In words, Volume = (circumference)(height)(thickness). This is easier to understand if you imagine the shell cut and rolled out to form a rectangular solid with length , height h, and width .

Based of this formula for the volume of one shell, we can come up with a formula for the volume of an entire solid made up of many shells. If f is a continuous function on the closed interval [a, b], then the volume of the solid obtained by rotating the graph of f from x = a to x = b about the y-axis is

(where ).

(where ).

Now for a sample problem:

about the line x = 3.

Solution:

First, visualize or sketch the graph. Either with your mind or with your calculator, you can see that the region bounded by the restrictions goes from x=0 to x=1 and lies between a smooth curve and the x-axis (similar to the example on page 458).

Then, draw or picture a vertical cross-section of the region. Rotated around x= 3, the shell has a radius of 3-x, a circumference of , and a height of y or .

Some websites:

## 1 Comments:

Hey thanks for the post on shells, I needed it.

I just had a sick week and before that I had two weeks of winter break, so my calculus got a little rusty (my teacher is testing me the first day I get back, eek).

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