When to use Washer, Shell or Disk

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Discussion Overview

The discussion revolves around the appropriate use of the Washer, Shell, and Disk methods for calculating volumes of solids of revolution using integrals. Participants seek clarity on when to apply each method and the factors influencing their choices.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • Some participants suggest that the choice between methods is based on personal preference and visualization of the solid.
  • Others argue that while any method can be used, certain problems may lend themselves more easily to one method over another.
  • One participant notes that the Disk method is typically straightforward when there is no hole in the center of the solid.
  • Another participant emphasizes that the Shell method can be easier depending on how the solid is described and suggests trying both methods on the same problem for practice.
  • There is mention of confusion arising from assignments that required using both the Washer and Shell methods for the same problems.
  • Technical details are provided about the integrals for each method, with the Shell method involving the integral of circumference times height, and the Washer method involving the difference of the areas of two disks.

Areas of Agreement / Disagreement

Participants express varying opinions on the ease of using different methods, indicating that there is no consensus on a definitive approach. Multiple competing views remain regarding the best method to use in different scenarios.

Contextual Notes

Participants highlight that the effectiveness of each method can depend on the specific problem setup and the integrals involved. There is an acknowledgment of potential confusion when switching between methods.

Who May Find This Useful

This discussion may be useful for students learning about volume calculations in calculus, particularly those grappling with the Washer, Shell, and Disk methods for solids of revolution.

Pseudo Statistic
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I'm curious, when am I supposed to use Washer, Shell or Disk method when trying to answer questions involving integrals and volume? Is there something specific I should look out for?
I just can't tell the difference.
Any help is appreciated.
Thanks.
 
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You can use whichever one you want. You can integrate using any shape. If you're doing the solids of revolution problems, I always thought that the shell method was easier.
 
Oh, I thought you weren't able to choose them on your own and each problem reverted to a specific one or something?
 
No, its a matter of how you find it easiest to visualize the solid.
 
Depending on how the solid is described, you'll sometimes find the shell method easier to integrate than the washer method and vice versa. If you set up the integral one way and you're finding it hard to evaluate, try using the other method. Try doing the same problems using both methods, it's a good way to get a feel for when one is preferable. Of course sometimes either way will be just as easy (or hard).
 
Oh good question! I've been wondering the same thing! Disk is usually pretty obvious b/c it's just a volume of solid revolution, usually there's no hole in the center or whatever the situation usually is.

Deciding between washer or shell can be tricky, and as posted above a lot of the time either method will work, but one might be easier. We actually had a homework assignment on the washer method, then the next homework assignment was to redo all the problems using the shell method! Talk about getting confused!

Some things to keep in mind is when using the shell method, you can think of a tiny segment rotating parallel to the axis of rotation (forming a cylinder). This integral takes the form of: integral(circumference*height*delta r).

When using washer, the segment is rotating perpendicular to the axis of rotation. This yields the integral where you have integral [(pi R^2)-(pi r^2)], where R is the radius of the larger disk or washer, and little r is (you guessed it!) the radius of the smaller or inner disk.

Happy integrating!
 

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