# Washer Method Vs Disk Method

The Washer Method and Disk Method are two techniques that are commonly used in calculus to determine the volume of a solid. These methods are used to calculate the volumes of solid objects that are obtained by rotating a two-dimensional shape about an axis in the 3D space.

The Washer Method

The Washer Method is a technique that is used to calculate the volume of a solid that is obtained by rotating a two-dimensional shape around the x or y-axis. When using this method, a cross-section of the solid is obtained by intersecting it with a plane that is perpendicular to the axis of rotation. The resulting cross-section is a washer-shaped figure with a hole in the middle.

The formula for calculating the volume of a solid using the Washer Method is given by:

V = π ∫[R(x)]^2 – [r(x)]^2 dx

where R(x) and r(x) represent the outer and inner radius of the washer, respectively.

The Disk Method

The Disk Method is another technique used to calculate the volume of a solid object that is obtained by rotating a two-dimensional shape around either the x or y-axis. With this method, the cross-section obtained by intersecting the solid with a plane perpendicular to the axis of rotation is a disk-shaped figure.

The formula for calculating the volume of a solid using the Disk Method is given by:

V = π ∫[R(x)]^2 dx or V = π ∫[R(y)]^2 dy

where R(x) or R(y) represents the radius of the disk.

Comparison between the Washer Method and Disk Method

Both the Washer Method and Disk Method are suitable for determining the volume of solid objects obtained by rotating two-dimensional shapes around the x or y-axis. However, there are some differences that make one method more suitable than the other in certain situations.

One of the main differences between the two methods is the shape of the cross-section that is obtained when intersecting the solid with a plane perpendicular to the axis of rotation. With the Washer Method, the cross-section is a washer-shaped figure, whereas with the Disk Method, the cross-section is a disk-shaped figure.

Another difference between the two methods is the range of the integral. With the Washer Method, the integral range is determined by the limits of the axis of rotation. In contrast, with the Disk Method, the integral range is determined by the limits of the variable (x or y) that define the shape of the solid.

In general, the Washer Method is more suitable for objects that have a hollow center, while the Disk Method is more suitable for objects that are solid throughout. For example, a sphere would be more suitable for the Disk Method, while a circular cylinder with a hole in the middle would be more suitable for the Washer Method.

FAQs

1. Can the Washer Method and Disk Method be used to find the volume of objects rotated around an inclined axis?

No, the Washer Method and Disk Method are designed to find the volume of objects rotated around the x or y-axis only.

2. Is it necessary to always rotate a two-dimensional shape around the x or y-axis?

No, it is not necessary. The shape can be rotated around other axes as well, but this will require a different method of calculation.

3. How can I know which method to use for a particular shape?

The method to use depends on the shape of the object being rotated. If the shape has a hollow center, then the Washer Method is more suitable. If the shape is solid throughout, then the Disk Method is more suitable.