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Solids of Revolution


The purpose of this lab is to use Maple to study solids of revolution. Solids of revolution are created by rotating curves in the x-y plane about an axis, generating a three dimensional object.


So far we have used the integral mainly to to compute areas of plane regions. It turns out that the definite integral can also be used to calculate the volumes of certain types of three-dimensional solids. The class of solids we will consider in this lab are called Solids of Revolution because they can be obtained by revolving a plane region about an axis.

As a simple example, consider the graph of the function $f(x) = x^2+1$ for $-2\leq x \leq 2$

If we take the region between the graph and the x-axis and revolve it about the x-axis, we obtain a solid

To help you in plotting surfaces of revolution, A Maple procedure called revolve has been written. The commands used to produce the graphs are shown below. The revolve procedure, as well as the RevInt, LeftInt, and LeftDisk procedures described below are all part of the CalcP7 package, which must be loaded first. The last line in the example below shows the optional argument for revolving the graph of $f(x)$ about the line $y=-2$ instead of the default $y=0$.

> with(CalcP7):
> f := x -> x^2+1;
> plot(f(x),x=-2..2,y=0..5);
> revolve(f(x),x=-2..2);
> revolve(f(x),x=-2..2,y=-2);

The revolve command has other options that you should read about in the help screen. For example, you can speed the command up by only plotting the surface generated by revolving the curve with the nocap argument, and you can also plot a solid of revolution formed by revolving the area between two functions. Try the following examples. (Note: The last example shows how to use revolve with a piecewise defined function using the piecewise command.)

> revolve({f(x),0.5},x=-2..2,y=-1);
> revolve(cos(x),x=0..4*Pi,y=-2,nocap);
> revolve({5,x^2+1},x=-2..2);
> g := x-> piecewise(x<0,-x+1/2,x^2-x+1/2);
> revolve(g(x),x=-1..2);

It turns out that the volume of the solid obtained by revolving the region between the graph and the $x$-axis about the $x$-axis can be determined from the integral

\begin{displaymath}\pi \int_{-2}^2 (x^2+1)^2 \, dx \end{displaymath}

to have the value $\displaystyle \frac{412}{15} \pi$. More generally, if you revolve the area under the graph of $g(x)$ for $a \leq x \leq
b$ about the $x$-axis, the volume is given by

\begin{displaymath}\pi \int_{a}^{b} (g(x))^2 \, dx\end{displaymath}

Where does this formula come from? The integral formula given above for the volume of a solid of revolution comes, as usual, from a limit process. Recall the rectangular approximations we used for plane regions. If you think of taking one of the rectangles and revolving it about the x-axis, you get a disk whose radius is the height $h$ of the rectangle and thickness is $\Delta x$, the width of the rectangle. The volume of this disk is $\pi h^2 \Delta x$. If you revolve all of the rectangles in the rectangular approximation about the x-axis, you get a solid made up of disks that approximates the volume of the solid of revolution obtained by revolving the plane region about the x-axis.

To help you visualize this approximation of the volume by disks, the LeftDisk procedure has been written. The syntax for this command is similar to that for revolve, except that the number of subintervals must be specified. The examples below produce approximations with five and ten disks. The latter approximation is shown in the graph below. The LeftInt command adds the disks.

> LeftDisk(f(x),x=-2..2,5);
> LeftInt(f(x),x=-2..2,5);
> LeftDisk(f(x),x=-2..2,10);
> LeftInt(f(x),x=-2..2,10);

Finding Volumes of Revolution

In order to calculate the volume of a solid of revolution use the int command implementing the formula above.

> Pi*int(f(x)^2,x=-2..2);
> evalf(Pi*int(f(x)^2,x=-2..2));


  1. For the function $\displaystyle f(x) = \sin(x-2)+1.1$ over the interval $-1 \leq x \leq 4$,
    Plot $f(x)$ over the given interval.
    Plot the approximation of the solid of revolution using LeftDisk with 12 disks.
    Plot the solid formed by revolving $f(x)$ about the $x$-axis using the revolve command.
    Find the exact volume of the solid of revolution and label your output exact.
    Find the number of subintervals needed to approximate the volume of the solid of revolution about the $x$-axis using LeftInt with error no greater than 0.1.
  2. Consider the funnel formed by revolving the curve $\displaystyle \frac{1}{2x}$ over the interval $[1,10]$.
    Plot the solid of revolution.
    Consider the same funnel over the interval $[1,b]$. Without plotting, find the volume of this funnel as $b \rightarrow \infty$. Use the command value instead of evalf.
  3. A doughnut is to be made by revolving a circle of radius $1$ centered at the origin about the $y=-2$ line.
    Write the equation of the circle using two functions for the top and bottom halves.
    Revolve your two functions about the $y=-2$ line. (To make it look like a donut right-click on the graph and click on scaling=constrained)
    To find the volume we need to shift the functions so the picture revolves about the x-axis. Rewrite your two functions shifting each 2 units.
    Revolve your new functions to show that it is the same donut just shifted.
    Find the volume of the donut.

next up previous
Next: About this document ... Up: lab_template Previous: lab_template
Jane E Bouchard