# Disc And Washer Method

The Disc and Washer Methods are two common techniques used in calculus to find the volume of a solid of revolution. These methods come in handy when we need to calculate the volume of an object formed by rotating a plane area around a certain axis. In this article, we will discuss the ins and outs of the Disc and Washer Methods, and provide a comparison between them.

Disc Method:

The Disc Method, also known as the cylindrical shell method, involves calculating the volume of a solid by slicing it into thin, cylindrical slices, adding up their volumes, and then integrating over the entire region.

To understand this method better, consider a function f(x) on an interval [a,b]. If we rotate this function about the x-axis, it will generate a solid of revolution. We can divide this solid into thin cylindrical shells that have the same height as dx and radius f(x). The volume of one of these shells can be calculated using the formula:

V = 2πxf(x)dx

We can add up the volumes of all these shells from a to b, which gives us the desired volume:

V = ∫a^b 2πxf(x)dx

Washer Method:

The Washer Method, also known as the method of rings, is another technique to find the volume of a solid of revolution. In this method, instead of slicing the solid into cylindrical shells, we slice it into washers.

To define this method, let us again consider the function f(x) on an interval [a,b]. If we rotate this function about the x-axis, it will generate a solid of revolution. We can divide this solid into thin washers that are infinitely small and have the same height as dx. Each washer can be generated by subtracting the volume of a smaller solid from a larger solid.

To calculate the volume of each washer, we need to find the volumes of two circular disks: one with radius F(x), and the other with radius f(x). The difference between these two disks is the volume of the washer at x. The formula to calculate the volume of the washer is given by:

V = π(F(x)^2 – f(x)^2) dx

Again, we can integrate this equation over the interval [a,b] to get the total volume:

V = ∫a^b π(F(x)^2 – f(x)^2) dx

Comparison:

Now let us compare the Disc and Washer Methods to see when we can use each of these methods.

Assuming that the function to be rotated is bounded by y = c, y = d, x = a, and x = b.

• When the axis of revolution is x-axis: In this case, the Disc Method is easier to use. This is because the radius of the cylindrical shell in the Disc Method is dx, which is aligned with the x-axis. Hence, the integration limits of the radius are automatically set to the interval [c,d]. However, in the Washer Method, the problem needs to be solved in two steps, which makes it more complicated.

• When the axis of revolution is y-axis: In this case, the opposite holds true. The Washer Method is easier to use because the radius of the circular disk in this method is dx, which is aligned with the y-axis. Hence, the integration limits of the radius are automatically set to the interval [a,b]. However, in the Disc Method, the problem must be solved in two steps, making it more complicated.

• When the area to be revolved is not completely symmetric around the axis of revolution: In this situation, we can use either method. However, the Disc Method may require more slices or shells to accurately calculate the volume, while the Washer Method may require more washers.

FAQs:

Q: What is the difference between the Disc and Washer Methods?

A: The Disc and Washer Methods are two techniques used to calculate the volume of a solid of revolution. The Disc Method involves slicing the solid into cylindrical shells, while the Washer Method involves slicing the solid into washers.

Q: When should I use the Disc Method?

A: The Disc Method is typically used when the axis of revolution is the x-axis, and the function to be rotated is bounded by y = c and y = d.

Q: When should I use the Washer Method?

A: The Washer Method is typically used when the axis of revolution is the y-axis, and the function to be rotated is bounded by x = a and x = b.

Q: Can I use either method if the area to be revolved is not symmetrical?

A: Yes, both methods can be used. However, the Disc Method may require more slices or shells, while the Washer Method may require more washers to accurately calculate the volume.

In conclusion, the Disc and Washer Methods provide us with two powerful techniques to calculate the volume of solids of revolution. While both methods have their strengths and weaknesses, choosing the right method depends on the situation at hand. By understanding the underlying principles of each method, we can become more adept at solving complex problems in calculus.