Virtual Lessons: Rotational Volume

To determine the volume of rotational symmetries there are two main protocols to follow: the Washer/Disk Method or the Shell Method. The washer method is a generalization of the disk method. Begin by sketching a 2D model, and DO NOT forget to include the cross-section. Most students are unable to solve these problems because they forgot to include this part. The cross-section is important for various reasons. Not only does it quickly help you determine which method to use, it also helps to measure heights and radii. 

To compute the radius, always measure FROM (not TO but FROM) the axis of the rotation. 

• Washer/Disk Method
  • Cross-Sections are taken perpendicular to the Axis of Rotation
• (Cylindrical) Shell Method
  • Cross-Sections are taken parallel to the Axis of Rotation