# Difference Between Disk And Washer Method

When studying calculus, one of the fundamental concepts is the integration of functions. Integration involves finding the area between a curve and an axis. However, it’s not always possible to find the exact formula for the area. This is where integral calculus becomes useful. Two methods used to estimate the area of a curve are the disk method and the washer method. In this article, we’ll discuss the differences between the two methods, compare them, and answer some frequently asked questions about these methods.

Disk Method

The disk method is a technique for finding the volume of an object by integrating the area of circles. The process involves selecting a cross-section of a solid figure, determining the radius, and then determining the area of the circle formed by that radius. The process is repeated for each radius along the entire length of the figure, then the areas are added together to arrive at the total volume.

The basic formula for the disk method is as follows:

V = π∫ [f(x)]^2 dx

Where f(x) is the function that represents the curve of the object to be measured, and dx represents the differential that sets the width of the disk.

For example, let’s find the volume of a solid formed by rotating the area between the vertical line x = 2 and the curve y = x^2 around the x-axis. First, we need to determine the limits of the integral, which are x = 0 and x = 2. Then, we can use the disk method:

V = π ∫[x^2]^2 dx from 0 to 2
V = π∫[x^4] dx from 0 to 2
V = π[(2^5/5) – (0^5/5)]
V = 32π/5

Washer Method

The washer method is a method of calculating the volume of a solid by breaking it down into smaller, thinner rings. Like the disk method, it involves selecting a cross-section of an object, but this time we must find the volume between two radii, one of which is smaller than the other.

The basic formula for the washer method is:

V = π∫[(R(x))^2 – (r(x))^2] dx

Where R(x) and r(x) represent the outer and inner radii of the figure, respectively.

For example, let’s find the volume of a solid formed by rotating the area between the vertical line x = 2 and the curves y = x^2 and y = 4-x^2 around the x-axis. First, we need to determine the limits of the integral, which are x = 0 and x = 2. Then, we can use the washer method:

V = π∫[(4-x^2)^2 – (x^2)^2] dx from 0 to 2
V = π∫[(16-8x^2+x^4) – (x^4)] dx from 0 to 2
V = π∫[16-8x^2] dx from 0 to 2
V = π[(16x – 8x^3)/3] from 0 to 2
V = (32π)/3

Comparison

Both the disk and washer methods are useful tools for calculating the volume of an object. However, there are clear differences between the two techniques. The disk method is best used for objects that are circular, while the washer method is useful for more complex shapes with a hole in the middle.

Moreover, the disk method uses only one radius of the figure, while the washer method uses two radii, one inside and the other outside the figure.

FAQs

1. What is the difference between a disk and a washer?
Ans. A disk is a solid figure formed by rotating a circle around an axis in its plane. A washer is a solid figure that has a hole in the middle, which is formed by rotating an area between two curves around an axis in its plane.

2. Can the disk and washer methods be used interchangeably?
Ans. No, the method selected depends on the type of object you want to measure. The disk method is best used for objects that are circular, while the washer method is useful for more complex shapes with a hole in the middle.

3. How do the disk and washer methods relate to integration?
Ans. Both the disk and washer methods involve integration. They require finding the volume of an object by integrating the area of circles (disk method) or smaller, thinner rings (washer method).

In conclusion, both the disk and washer methods are integral to calculating the volume of objects. They both involve integration, and both have their own strengths and limitations. By understanding the differences between the two methods, calculus students can approach questions on volume with greater confidence and precision.