Spin a region about an axis and every slice through it is a circle. Adding up the areas of those circles is an integral: pi times the radius squared, all the way along.
The region under y=x from x=0 to x=3 is spun about the x-axis. Find the volume.
Each slice is a circle of radius y, which is x.
So the volume is pi times the integral of x squared from 0 to 3.
That is pi times 27 over 3, which is 9 pi.
The answer is 9π.
Where students go wrong
Integrating the radius instead of its square. Each slice is a circle, and the area of a circle is pi r squared, so the function gets squared before it is integrated.
Also called volumes of revolution, disk method, washer method, solid of revolution or rotating about the x-axis.
Questions about these worksheets
Yes. Every volume by disks and washers sheet is free to print and free to download as a PDF. There is no account to make, no email to hand over and no limit on how many you take.
Yes. The answer key prints on a second page, and you can choose whether it shows each question beside its answer or just the answers in a list.
Yes. Print straight from the page, or download the PDF and print that. The sheet is laid out for paper rather than squeezed off a screen, so there is room to work under each question, and the answer key comes out on its own page.
Calculus is usually taken around 12th grade or a first college course, though schools vary and plenty of students meet it earlier or later. Pick the difficulty rather than the grade: the easy level suits a first lesson on it, the hard level suits review before a test.
Yes, as many as you like. The questions are built fresh each time rather than picked from a fixed set of files, so pressing Generate gives a new sheet. That is what you want for a second class, a retake, or two students sitting next to each other.
That is the closest comparison, and Kuta Software is good software. It is also paid software you install on a computer. These worksheets run in a browser tab for free, with no account and nothing to install. Like Kuta, the volume by disks and washers questions are generated when you ask for them rather than pulled from a fixed set of files, so two classes never get the same sheet, and the answer key comes with it.
Yes. There are three levels, and you can tick more than one for a sheet that mixes them, in which case the questions come out easiest first. The harder levels are not just larger numbers: they ask for something the easy ones do not.
Integrating the radius instead of its square. Each slice is a circle, and the area of a circle is pi r squared, so the function gets squared before it is integrated.