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How to use the Disk Method to find the volume of a solid that is obtained by rotating an appropriate plane region about a line.

The formula for the volume of the solid of revolution that has disks as its cross-section is given by
  • , if the axis of rotation is the x-axis.
  • , if the axis of rotation is the y-axis.
Observe that R (x) and R (y) are the radii of the disks drawn at x and y respectively.
*Disk Method is an application of the method of slicing.

Professionals, like architects, Pharmaceutical companies , and engineers often need to calculate the volume of objects. Here are some examples.

Lathes are machines that are used to fabricate a diverse set of objects that have circular cross sections ranging from a piece of jewelry by craftsmen to a gun barrel or bullets by military engineers. Sometimes the volume of an object needs to be known before it can be shaped by the lathe. For instance, when designing a bullet that can fit into a certain size of the gun barrel we need to know its specific volume.
Pharmaceutical companies can determine the volume of each pill to find out the correct dosage and also the number of pills a bottle can hold based on its volume.
Symphony drums are hemispherical or parabolic in shape. The shape of the ball determines its tone. Calculating the volume would allow you to create drums of different pitches.

Click on the tabs on the right to watch videos on rotating a region about the x-axis and y-axis.

VOLUMES OF REVOLUTION - DISK METHOD
To rotate grid, click and drag
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REVOLVE REGION
# OF DISKS
AXIS OF REVOLUTION
x y
Region 1   y=-1012122-x2, y=0
Volume   abπy2dx0
Volume   cdπx2dy0
Region 2   y=(x-3)2 when x3,x=0 y=4
Volume   abπy2dx0
Volume   cdπx2dy0
?
Region 3   y=x2-2x+2, x=0, x=3 and y=0
Volume   abπy2dx0
Volume   cdπx2dy0
?
Number of Disks:0Width of Disks:0.00Sum of Volume of Disks  0.00   units3
Note
Note that the disk method is not applicable here since not all the slices of the solid generated are disks. However, you will be able to compute its volume using washer method which will be covered in the next module.
Select any of the regions from the three options given at the bottom left-hand corner of the screen. Rotate the region around either x or y axis to obtain the solid and answer the following questions.
1. When you move the second slider (# of disks) from left to right, notice that the partitioning rectangles show up. Which of the following is true about these rectangles? Check all that apply.
2. Revolve the selected region about the x-axis and then move the second slider to obtain 10 partitioning disks. What is the width of each disk? Check all that apply.
3. Revolve the selected region about the y-axis to obtain a mixing bowl filled to the rim with cake batter. Set up an integral to compute the volume of the bowl. Check all that apply.
4. Which of the following axes of rotation would yield cross-sections of the solid of revolution as only disks?
5. Select the axis of revolution as the y-axis. Move the second slider (# of disks) to obtain 2 partitioning rectangles. Note that we are using the midpoint method to partition the region. What are the lengths of the two rectangles?
6. Revolve the selected region about the y-axis and then move the second slider to obtain 2 partitioning disks. What are the radii of the two disks?
7. Which of the following axes of rotation would yield cross-sections of the solid of revolution as only disks?
8. Set the axis of revolution to be the y-axis. Move the second slider (# of disks) to obtain 5 partitioning rectangles of equal widths. What is the width of each rectangle?
9. Set up an integral to compute the volume of the solid of revolution obtained by rotating the selected region about the x-axis.
Result