# Shell Method Vs Disk Method

When it comes to calculus, there are specific methods used to calculate areas and volumes of different shapes. The two most commonly used methods for finding volumes of three-dimensional shapes are the shell method and the disk method. Both methods have their advantages and disadvantages depending on the shape being analyzed. In this article, we will discuss the differences between the shell method and the disk method and their applications.

What is the Shell Method?

The shell method is a technique used to calculate the volume of a solid of revolution by integrating cylindrical shells. Solid of revolution refers to a three-dimensional shape obtained by rotating a two dimension shape around a central axis.

The shell method is most commonly used to calculate the volume of a cylinder or a frustrum, a cone with the top cut off. The formula for the shell method is V=2π ∫ x( f(x)-g(x))dx, where V represents the volume of the solid, f(x) and g(x) are the two equations of the curves that enclose the solid, and x is the variable of integration. The integral is taken between the bounds of the solid.

For a better understanding of the shell method, let’s take the example of finding the volume of a cylinder with radius r and height h, using the shell method.

First, we will slice the cylinder into small circular strips parallel to its base. When we place these strips at the top of each other, they form a cylindrical shell.

Next, we will calculate the volume of each cylindrical shell. The volume of one cylindrical shell can be calculated by multiplying its height, which is r, and its circumference, which is 2π. Therefore, the volume of one cylindrical shell will be 2πr.

Now, to calculate the volume of the entire cylinder, we will add up the volumes of all these cylindrical shells. The set of infinite shells will be 2πrh if the height of the cylinder is h.

What is the Disk Method?

The disk method is another method used to calculate the volume of a solid of revolution. This method involves slicing the solid into thin disk-shaped segments or slices, then adding up the volume of each slice. This method is most commonly used in calculating the volume of a sphere or a cone.

To understand the disk method, let’s take the example of finding the volume of a sphere with radius r.

First, we will slice the sphere into thin disks, parallel to its base. We begin by cutting the sphere in half, creating a cross-section. Next, we will slice the half-sphere into circular disks perpendicular to the base.

The volume of one disk can be calculated by multiplying the area of its base, which is πr², by its thickness, which is smaller than the radius. Hence, the volume of one disk is πr²x, where x is the thickness of the disk.

Now to get the volume of the whole sphere, the set of infinite disks, we will integrate the volume of one disk using the definite limits of the function, which will be written as V=π∫(R²-x²)dx.

The Difference between Shell Method and Disk Method

Even though both methods can be used to calculate the volume of a solid of revolution, there are significant differences between them.

In the shell method, the shape is divided into cylindrical shells that are integrated along the axis of rotation. As a result, this method works best when the axis of revolution is on the side of the shape. The shell method is also ideal when dealing with shapes that have difficult cross-sections, such as cut-out sections or asymmetrical shapes.

On the other hand, the disk method divides the shape into a series of slices, which are then measured and added together. This method works best when the axis of revolution is located in the center of the shape. The disk method is ideal for dealing with shapes that have a clear circular cross-section.

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Conclusion

The shell method and the disk method are both essential techniques used in calculus to calculate the volume of a solid of revolution. The shell method is more appropriate when the axis of rotation is on the side of the shape, while the disk method is ideal when the axis of rotation is in the center of the solid. When dealing with complex shapes with varying cross-sections, the shell method proves more useful. The disk method, on the other hand, excels at handling shapes with clear circular cross-sections. Understanding the differences between these two methods can be helpful in calculating volumes of three-dimensional shapes.