## What Is The covering Method

The shell method, periodically referred to as the an approach of cylindrical shells, is another method commonly supplied to find the volume the a hard of revolution.

You are watching: When to use washer or shell method

So, the idea is that we will certainly revolve cylinders about the axis of revolution rather 보다 rings or disks, as previously done utilizing the decaying or washer methods.

How walk this work?

Geometrically, we understand that the surface area that a cylinder is found by multiplying the circumference of the circular basic times the height of the cylinder.

\beginequationS A=2 \pi r h\endequation

But this well known formula from geometry doesn’t take right into account the thickness the the cylinder that is created.

This means that every cylinder the revolves approximately the axis has a thickness, w. So, if us let p represents the average radius, or the displacement from the axis of rotation, and also the h represent the cylinder’s elevation or length, climate the surface area that one cylinder is the product the the circumference times the height times the thickness.

Surface Area of Cylinder

\beginequationS A=\underbrace(\text one )_2 \pi p \underbrace(\text height )_h \underbrace(\text thickness )_w=2 \pi ns h \Delta x\endequation

And if we revolve an infinite number of cylinders, climate the an outcome is the volume of the solid. And also we amount an infinite variety of cylinders by

\beginequation\lim _n \rightarrow \infty \sum_i=1^n 2 \pi(\text radius )(\text elevation )(\text thickness )=\lim _n \rightarrow \infty \sum_i=1^n 2 \pi ns h \Delta x\endequation

## Disk method Vs shell Method

As the graphic below nicely illustrates, there is a considerable difference between the disk method and the shell method.

The Shell an approach vs Disk method (X-Axis)

The Shell technique vs Disk an approach (Y-Axis)

For the disk/washer method, the slice is perpendicular to the axis the revolution, whereas, because that the shell method, the slice is parallel to the axis of revolution.

### Example

Okay, for this reason let’s see the shell an approach in action to make sense of this brand-new technique.

Find the volume of the solid acquired by rotating about the x-axis the region bounded between

\beginequationy=x^2, y=0, x=0, \text and also x=4\endequation

First, let’s graph the an ar and discover all points of intersection.

Find The Volume that The Solid generated By Revolving The region Bounded

Now, let’s calculate the volume making use of the decaying (washer) method and the shell method, next by side, and also see how they compare.

Disk an approach Vs covering Method

Isn’t that awesome to view that both approaches yield the same result!

Cool!

But, if we can use either technique, exactly how do you understand when to usage the covering or disc method?

So, together we saw through the instance above, recognize volume making use of the disk or washer an approach will produce the same an outcome when calculating utilizing the covering method. Consequently, the techniques are interchangeable, and also it comes down to an individual preference regarding which integration an approach you utilize. Yep, you get to choose which method you prefer better.

However, there are times once the shell method is the clear winner, together the disk method is insufficient.

### Example

For instance, expect we are asked to find the volume of the solid acquired by rotating about the y-axis the region bounded by

\beginequationy=2 x^2-x^3 \text and y=0\endequation

First, let’s graph the region and find all point out of intersection.

Shell technique Formula (Around Y-Axis)

And we quickly notification that if we tried to usage the washer method, our “top” (outer) role is the same as the “bottom” (inner) function, which means they would remove each other!

Therefore, rather than making use of rectangles perpendicular to the axis the revolution, we need to use rectangles parallel to the axis that rotation by making use of the shell method.

See more: Gonna Need A Little Less Edge On That, Gonna Need A Little Less Edge

Shell an approach (Finding Radius and Height)

\beginequation\beginarraylV=2 \pi \int_a^b p(x) h(x) d x \\V=2 \pi \int_0^2(x-0)\left(\left(2 x^2-x^3\right)-0\right) d x \\V=2 \pi \int_0^2 x\left(2 x^2-x^3\right) d x=2 \pi \int_0^16\left(2 x^3-x^4\right) d x \\V=\frac16 \pi5\endarray\endequation

Gosh, that means we to be able to take it a shaded an ar and revolve it around an axis to produce a solid!

Volume Of heavy Of revolution For Cylinder

Approximation the N Cylinders

See, this an approach is supervisor handy and downright necessary!

## Summary

Together, in this video lesson, we will certainly walk through many examples in information so that you will have actually a solid understanding of how and also when to usage this shell an approach to good success.