# Solids of Revolution by Disks

We can have a function, like this one:

And revolve it around the x-axis like this:

To find its **volume** we can **add up a series of disks**:

Each disk's face is a circle:

The area of a circle is **π** times radius squared:

A = **π** r^{2}

And the radius **r** is the value of the function at that point **f(x)**, so:

A = **π** f(x)^{2}

And the **volume** is found by summing all those disks using Integration:

**π**f(x)

^{2}dx

And that is our formula for **Solids of Revolution by Disks**

In other words, to find the volume of revolution of a function f(x): **integrate pi times the square of the function**.

### Example: A Cone

Take the very simple function **y=x** between 0 and b

Rotate it around the x-axis ... and we have a cone!

The radius of any disk is the function f(x), which in our case is simply **x**

What is its volume? **Integrate pi times the square of the function x** :

**π**(x)

^{2}dx

First, let's have our **pi outside** (yum).

Seriously, it is OK to bring a constant outside the integral:

**π**

^{2}dx

Using Integration Rules we find the integral of x^{2} is **x ^{3}/3 + C**

To calculate this definite integral, we calculate the value of that function for **b** and for **0** and subtract, like this:

Volume = **π** (b^{3}/3 − 0^{3}/3)

= **π** b^{3}/3

Volume = \frac{1}{3} π r^{2} h

When both **r=b** and **h=b** we get:

Volume = \frac{1}{3} π b^{3}

As an interesting exercise, why not try to work out the more general case of any value of r and h yourself?

We can also rotate about other lines, such as x = −1

### Example: Our Cone, But About x = −1

So we have this:

Rotated about x = −1 it looks like this:

The cone is now bigger, with its sharp end cut off (a *truncated cone*)

Let's draw in a sample disk so we can work out what to do:

OK. Now what is the radius? It is our function **y=x** plus an extra **1**:

y = x + 1

Then **integrate pi times the square of that function**:

**π**(x+1)

^{2}dx

**Pi outside**, and expand (x+1)^{2} to x^{2}+2x+1 :

**π**

^{2}+2x+1) dx

Using Integration Rules we find the integral of x^{2}+2x+1 is **x ^{3}/3 + x^{2} + x + C**

And going between **0** and **b** we get:

Volume = **π** (b^{3}/3+b^{2}+b − (0^{3}/3+0^{2}+0))

= **π** (b^{3}/3+b^{2}+b)

Now for another type of function:

### Example: The Square Function

Take **y = x ^{2}** between x=0.6 and x=1.6

Rotate it around the x-axis:

What is its volume? **Integrate pi times the square of x ^{2}**:

**π**(x

^{2})

^{2}dx

Simplify by having pi outside, and also (x^{2})^{2} = x^{4} :

**π**

^{4}dx

The integral of x^{4} is **x ^{5}/5 + C**

And going between 0.6 and 1.6 we get:

Volume = **π** ( 1.6^{5}/5 − 0.6^{5}/5 )

≈ 6.54

Can you rotate **y = x ^{2}** about x = −1 ?

## In summary:

- Have pi outside
- Integrate the
**function squared** - Subtract the lower end from the higher end

## About The Y Axis

We can also rotate about the Y axis:

### Example: The Square Function

Take y=x^{2}, but this time using the **y-axis** between y=0.4 and y=1.4

Rotate it around the **y-axis**:

And now we want to integrate in the y direction!

So we want something like **x = g(y)** instead of y = f(x). In this case it is:

x = √(y)

Now **integrate pi times the square of √(y) ^{2}** (and dx is now

**dy**):

**π**√(y)

^{2 }dy

Simplify with pi outside, and √(y)^{2} = y :

**π**

The integral of y^{} is y^{2}/2

And lastly, going between 0.4 and 1.4 we get:

Volume = **π** ( 1.4^{2}/2 − 0.4^{2}/2 )

≈ 2.83...

## Washer Method

Washers: Disks with Holes

What if we want the volume **between two functions**?

### Example: Volume between the functions **y=x** and **y=x**^{3} from x=0 to 1

^{3}

These are the functions:

Rotated around the x-axis:

The disks are now "washers":

And they have the area of an annulus:

In our case **R = x** and **r = x ^{3}**

In effect this is the **same as the disk method**, except we subtract one disk from another.

And so our integration looks like:

**π**(x)

^{2 }−

**π**(x

^{3})

^{2 }dx

Have pi outside (on both functions):

**π**

^{2 }−

**(x**

^{3})

^{2}dx

Simplify:

**π**

^{2 }− x

^{6}dx

The integral of x^{}^{2} is x^{3}/3 and the integral of x^{6}^{} is x^{7}/7

And so, going between 0 and 1 we get:

Volume = **π** [ (1^{3}/3 − 1^{7}/7 ) − (0−0) ]

≈ 0.598...

So the Washer method is like the Disk method, but with the inner disk subtracted from the outer disk.