Volumes of Revolution: Cylindrical Shells
Learning Objectives
- Calculate the volume of a solid of revolution by using the method of cylindrical shells.
- Compare the different methods for calculating a volume of revolution.
In this section, we examine the method of cylindrical shells, the final method for finding the volume of a solid of revolution. We can use this method on the same kinds of solids as the disk method or the washer method; however, with the disk and washer methods, we integrate along the coordinate axis parallel to the axis of revolution. With the method of cylindrical shells, we integrate along the coordinate axis perpendicular to the axis of revolution. The ability to choose which variable of integration we want to use can be a significant advantage with more complicated functions. Also, the specific geometry of the solid sometimes makes the method of using cylindrical shells more appealing than using the washer method. In the last part of this section, we review all the methods for finding volume that we have studied and lay out some guidelines to help you determine which method to use in a given situation.
The Method of Cylindrical Shells
Again, we are working with a solid of revolution. As before, we define a region bounded above by the graph of a function below by the and on the left and right by the lines and respectively, as shown in (Figure)(a). We then revolve this region around the -axis, as shown in (Figure)(b). Note that this is different from what we have done before. Previously, regions defined in terms of functions of were revolved around the or a line parallel to it.
As we have done many times before, partition the interval using a regular partition, and, for choose a point Then, construct a rectangle over the interval of height and width A representative rectangle is shown in (Figure)(a). When that rectangle is revolved around the -axis, instead of a disk or a washer, we get a cylindrical shell, as shown in the following figure.
To calculate the volume of this shell, consider (Figure).
The shell is a cylinder, so its volume is the cross-sectional area multiplied by the height of the cylinder. The cross-sections are annuli (ring-shaped regions—essentially, circles with a hole in the center), with outer radius and inner radius Thus, the cross-sectional area is The height of the cylinder is Then the volume of the shell is
Note that so we have
Furthermore, is both the midpoint of the interval and the average radius of the shell, and we can approximate this by We then have
Another way to think of this is to think of making a vertical cut in the shell and then opening it up to form a flat plate ((Figure)).
In reality, the outer radius of the shell is greater than the inner radius, and hence the back edge of the plate would be slightly longer than the front edge of the plate. However, we can approximate the flattened shell by a flat plate of height width and thickness ((Figure)). The volume of the shell, then, is approximately the volume of the flat plate. Multiplying the height, width, and depth of the plate, we get
which is the same formula we had before.
To calculate the volume of the entire solid, we then add the volumes of all the shells and obtain
This leads to the following rule for the method of cylindrical shells.
Rule: The Method of Cylindrical Shells
Let be continuous and nonnegative. Define as the region bounded above by the graph of below by the on the left by the line and on the right by the line Then the volume of the solid of revolution formed by revolving around the -axis is given by
Now let’s consider an example.
The Method of Cylindrical Shells 1
Define as the region bounded above by the graph of and below by the over the interval Find the volume of the solid of revolution formed by revolving around the
Answer:
First we must graph the region and the associated solid of revolution, as shown in the following figure.
Then the volume of the solid is given by
Define R as the region bounded above by the graph of and below by the -axis over the interval Find the volume of the solid of revolution formed by revolving around the
units3
The Method of Cylindrical Shells 2
Define R as the region bounded above by the graph of and below by the over the interval Find the volume of the solid of revolution formed by revolving around the
Answer:
First graph the region and the associated solid of revolution, as shown in the following figure.
Then the volume of the solid is given by
Define as the region bounded above by the graph of and below by the over the interval Find the volume of the solid of revolution formed by revolving around the
Answer:
units3
Hint
Use the process from (Figure).
As with the disk method and the washer method, we can use the method of cylindrical shells with solids of revolution, revolved around the when we want to integrate with respect to The analogous rule for this type of solid is given here.
Rule: The Method of Cylindrical Shells for Solids of Revolution around the -axis
Let be continuous and nonnegative. Define as the region bounded on the right by the graph of on the left by the below by the line and above by the line Then, the volume of the solid of revolution formed by revolving around the is given by
The Method of Cylindrical Shells for a Solid Revolved around the -axis
Define as the region bounded on the right by the graph of and on the left by the for Find the volume of the solid of revolution formed by revolving around the -axis.
Answer:
First, we need to graph the region and the associated solid of revolution, as shown in the following figure.
Label the shaded region Then the volume of the solid is given by
Define as the region bounded on the right by the graph of and on the left by the for Find the volume of the solid of revolution formed by revolving around the
Answer:
units3
Hint
Use the process from (Figure).
For the next example, we look at a solid of revolution for which the graph of a function is revolved around a line other than one of the two coordinate axes. To set this up, we need to revisit the development of the method of cylindrical shells. Recall that we found the volume of one of the shells to be given by
This was based on a shell with an outer radius of and an inner radius of If, however, we rotate the region around a line other than the we have a different outer and inner radius. Suppose, for example, that we rotate the region around the line where is some positive constant. Then, the outer radius of the shell is and the inner radius of the shell is Substituting these terms into the expression for volume, we see that when a plane region is rotated around the line the volume of a shell is given by
As before, we notice that is the midpoint of the interval and can be approximated by Then, the approximate volume of the shell is
The remainder of the development proceeds as before, and we see that
We could also rotate the region around other horizontal or vertical lines, such as a vertical line in the right half plane. In each case, the volume formula must be adjusted accordingly. Specifically, the in the integral must be replaced with an expression representing the radius of a shell. To see how this works, consider the following example.
A Region of Revolution Revolved around a Line
Define as the region bounded above by the graph of and below by the over the interval Find the volume of the solid of revolution formed by revolving around the line
Answer:
First, graph the region and the associated solid of revolution, as shown in the following figure.
Note that the radius of a shell is given by Then the volume of the solid is given by
Define as the region bounded above by the graph of and below by the over the interval Find the volume of the solid of revolution formed by revolving around the line
Answer:
units3
Hint
Use the process from (Figure).
For our final example in this section, let’s look at the volume of a solid of revolution for which the region of revolution is bounded by the graphs of two functions.
A Region of Revolution Bounded by the Graphs of Two Functions
Define as the region bounded above by the graph of the function and below by the graph of the function over the interval Find the volume of the solid of revolution generated by revolving around the
Answer:
First, graph the region and the associated solid of revolution, as shown in the following figure.
Note that the axis of revolution is the so the radius of a shell is given simply by We don’t need to make any adjustments to the -term of our integrand. The height of a shell, though, is given by so in this case we need to adjust the term of the integrand. Then the volume of the solid is given by
Define as the region bounded above by the graph of and below by the graph of over the interval Find the volume of the solid of revolution formed by revolving around the
Answer:
units3
Hint
Use the process from (Figure).
Which Method Should We Use?
We have studied several methods for finding the volume of a solid of revolution, but how do we know which method to use? It often comes down to a choice of which integral is easiest to evaluate. (Figure) describes the different approaches for solids of revolution around the It’s up to you to develop the analogous table for solids of revolution around the
Let’s take a look at a couple of additional problems and decide on the best approach to take for solving them.
Selecting the Best Method
For each of the following problems, select the best method to find the volume of a solid of revolution generated by revolving the given region around the and set up the integral to find the volume (do not evaluate the integral).
- The region bounded by the graphs of and the
- The region bounded by the graphs of and the
Answer:
- First, sketch the region and the solid of revolution as shown.
we would have to break the integral into two pieces, because we have different functions bounding the region over and In this case, using the disk method, we would have
If we used the shell method instead, we would use functions of to represent the curves, producingNeither of these integrals is particularly onerous, but since the shell method requires only one integral, and the integrand requires less simplification, we should probably go with the shell method in this case.
Looking at the region, if we want to integrate with respect to
- First, sketch the region and the solid of revolution as shown.
Select the best method to find the volume of a solid of revolution generated by revolving the given region around the and set up the integral to find the volume (do not evaluate the integral): the region bounded by the graphs of and
Answer:
Use the method of washers;
Hint
Sketch the region and use (Figure) to decide which integral is easiest to evaluate.
Key Concepts
- The method of cylindrical shells is another method for using a definite integral to calculate the volume of a solid of revolution. This method is sometimes preferable to either the method of disks or the method of washers because we integrate with respect to the other variable. In some cases, one integral is substantially more complicated than the other.
- The geometry of the functions and the difficulty of the integration are the main factors in deciding which integration method to use.
Key Equations
- Method of Cylindrical Shells
For the following exercise, find the volume generated when the region between the two curves is rotated around the given axis. Use both the shell method and the washer method. Use technology to graph the functions and draw a typical slice by hand.
1. [T] Over the curve of and rotated around the
2. [T] Under the curve of rotated around the
Answer: units3
3. [T] Over the curve of rotated around the
4. [T] Under the curve of rotated around the
Answer: units3
5. [T] Under the curve of rotated around the
6. [T] Under the curve of rotated around the
Answer: units3
For the following exercises, use shells to find the volumes of the given solids. Note that the rotated regions lie between the curve and the and are rotated around the
7.
8.
Answer:
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9.
10.
Answer:
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11.
12.
Answer:
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13.
14.
Answer:
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15.
16.
Answer:
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For the following exercises, use shells to find the volume generated by rotating the regions between the given curve and around the
17.
18.
Answer:
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19.
20.
Answer:
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21.
22.
Answer:
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23.
24.
Answer:
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25.
26.
Answer:
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For the following exercises, find the volume generated when the region between the curves is rotated around the given axis.
27. rotated around the
28. rotated around the
Answer:
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29. rotated around the
30. rotated around the line
Answer:
units3
31. rotated around the line
32. rotated around the
Answer:
units3
33. rotated around the line
34. rotated around the
Answer:
units3
35. rotated around the line
36. [T] Left of right of around the
Answer:
0.9876 units3
For the following exercises, use technology to graph the region. Determine which method you think would be easiest to use to calculate the volume generated when the function is rotated around the specified axis. Then, use your chosen method to find the volume.
37. [T] and rotated around the
38. [T] rotated around the
Answer: units3
39. [T] rotated around the
40. [T] rotated around the
Answer: units3
41. [T] rotated around the
42. [T] rotated around the
Answer: units3
43. [T] and rotated around the
44. [T] rotated around the
Answer: 15.9074 units3
For the following exercises, use the method of shells to approximate the volumes of some common objects, which are pictured in accompanying figures.
45. Use the method of shells to find the volume of a sphere of radius
46. Use the method of shells to find the volume of a cone with radius and height
Answer:
units3
47. Use the method of shells to find the volume of an ellipse rotated around the
48. Use the method of shells to find the volume of a cylinder with radius and height
Answer:
units3
49. Use the method of shells to find the volume of the donut created when the circle is rotated around the line
50. Consider the region enclosed by the graphs of and What is the volume of the solid generated when this region is rotated around the Assume that the function is defined over the interval
Answer:
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51. Consider the function which decreases from to Set up the integrals for determining the volume, using both the shell method and the disk method, of the solid generated when this region, with and is rotated around the Prove that both methods approximate the same volume. Which method is easier to apply? (Hint: Since is one-to-one, there exists an inverse )
Glossary
- method of cylindrical shells
- a method of calculating the volume of a solid of revolution by dividing the solid into nested cylindrical shells; this method is different from the methods of disks or washers in that we integrate with respect to the opposite variable
Hint
Use the procedure from (Figure).