Disk method

Definition

The disk method is a technique used to find the volume of a solid of revolution by slicing the solid into thin disks perpendicular to the axis of rotation. Each disk's volume is calculated using the formula for the volume of a cylinder, and then these volumes are summed (integrated) across the entire solid. This method is particularly useful for solids formed by rotating regions bounded by curves around an axis.

5 Must Know Facts For Your Next Test

1. The volume of each disk is determined by the formula $$V = \pi r^2 h$$, where 'r' is the radius and 'h' is the thickness of the disk.
2. When using the disk method, the integral bounds correspond to the limits of the region being revolved.
3. The disk method can be applied when rotating around both the x-axis and y-axis, depending on how the functions are defined.
4. To apply the disk method, it's important to express the radius of each disk as a function of the variable being integrated.
5. The disk method provides an accurate way to calculate volumes for solids that can be represented by continuous functions.

Review Questions

• How do you set up an integral to find the volume of a solid using the disk method?
  • To set up an integral for finding the volume using the disk method, first identify the function that defines the shape being revolved around an axis. Then, determine the bounds of integration based on where the region starts and ends. The volume can be calculated using the integral $$V = \pi \int_{a}^{b} [f(x)]^2 \, dx$$ or $$V = \pi \int_{c}^{d} [g(y)]^2 \, dy$$, depending on whether you're revolving around the x-axis or y-axis.
• Discuss how you would modify your approach if you needed to use the washer method instead of the disk method.
  • If you need to use the washer method instead of the disk method, you will consider cases where there is an inner radius as well as an outer radius due to a hole in the solid. The volume for each washer is calculated as $$V = \pi (R^2 - r^2) h$$, where 'R' is the outer radius and 'r' is the inner radius. The integral would then combine these two functions, leading to a total volume formula that subtracts the volume represented by the inner radius from that of the outer radius.
• Evaluate how understanding the disk method enhances your ability to solve complex volume problems involving solids of revolution.
  • Understanding the disk method enhances your ability to tackle complex volume problems because it provides a systematic approach to visualizing and calculating volumes. By breaking down a solid into infinitesimally small disks, you simplify what might initially seem like an overwhelming problem into manageable parts. This method not only reinforces your understanding of integration but also helps connect geometric intuition with analytical techniques, making it easier to apply these concepts in varied scenarios involving solids formed by curves.

Related terms

solid of revolution: A three-dimensional shape created by rotating a two-dimensional shape around an axis.

washer method:

A variation of the disk method used when the solid has a hole in it, allowing for the calculation of volumes with an inner and outer radius.

integration:

A mathematical operation that combines small parts to find the whole, often used in calculating areas under curves and volumes of solids.