## Rotating Volumes with the Cylinder/Shell Method

Similar to using the disk or washer method, we will use the cylinder method to find the volume of a solid. Specifically, it’s used when we rotate a function or region around an axis of rotation. In fact, most problems that require finding the volume of a solid of rotation can use the disk/washer method or the cylinder method. However, one will usually be significantly easier.

I’ll explain what I mean by this with an example.

## Example 1

Find the volume of the solid resulting from rotating the area bound between $$y=x^2-2x+2$$, $$y=0$$, $$x=1$$, and $$x=2$$ about the y-axis.

Although we are using a different method here we will follow the same 4 step process as I did with the disk method and washer method.

### 1. Graph the 2-D functions

This is generally a good idea with any type of problem: draw out whatever is being described in the problem. This helps to visualize whats going on in the problem and what exactly we are trying to measure. It can also help us decide if the answer we end up with is actually a reasonable one.

I would recommend trying to graph all of the functions listed by hand, but I’ll do this using Desmos. You can see the graph of the functions below with the bounded region shaded gray.

### 2. Rotate the 2-D area around the given axis

Again, we want to visualize the 3-D figure whose volume we are trying to find. To do this we want to imagine rotating the described area around the axis of rotation.

I like to imagine the area actually coming off the page and rotating around our axis of rotation, which is the y-axis in this case. Doing this would create a cylinder-like, round figure. Try sketching this rotation and the resulting figure. It may be helpful in the next step to have this sketch.

If needed, Wolfram Alpha can always be used to create a visualization of this figure.

### 3. Setting up the integral

This is the part where things start to get a bit different using the cylinder method than they were with the disk/washer method.

In order to make sense of the integral we need to set up here, let’s think about what we’re doing differently. With the disk/washer methods we were stacking many very thin disks on top of each other with the same thickness and varying radii to create our figure. This created a stack of cylinders whose volume we could find and add together.

The cylinder shell method is a bit different. Here we need to imagine just the outer shell of a cylinder that is very very very thin. We will stack many of these very thin shells inside of each other to create our figure. Each shell will have the same thickness, but all with different heights depending on where it is in the figure.

What we need to figure out is a formula for the volume of one of these shells, then the integral will be able to go through each shell and add up all of their volumes.

#### What would this look like?

Before we think about creating a function that we will need to integrate I want to take a moment to describe what one of these shells would look like. Imagine forming the outer shell of a cylinder with a piece of paper. All you would need to do is roll up the paper and it would create a cylinder shell. But what’s interesting about this is the cylinder shell came from a rectangle, or more accurately a very very thin rectangular prism. It might look something like this.

Since we are imagining finding the volume of an infinitely thin cylinder, these three figures would have the same volume. The fact that they’re infinitely thin means that the curvature won’t impact the volume of the shell. So to find the volume of the cylindrical shell, we can instead find the volume of a rectangular prism with the same dimensions. This is a much easier exercise to imagine since the volume of a rectangular prism is simply $$V= \ length \cdot width \cdot height.$$

#### But how does this relate to the cylinder?

What we need to think about now is how the dimensions of the rectangular prism translate to dimensions on the cylindrical shell. Looking back up to the drawing above, if you imagine the rectangle curling into a cylinder you can see the long side of the rectangle bends into the top and bottom of the cylinder. This would be the circumference of the cylinder.

In the above drawing we can also see that the shorter side of the rectangle lines up nicely with the height of the cylinder.

And lastly, the very very very thin thickness of the shell and the rectangular prism would clearly correspond with each other.

So as a result, the $$V= \ length \cdot width \cdot height$$ of the rectangular prism would be the same as $$V = circumference \cdot thickness \cdot height$$ for the cylindrical shell.

#### Putting it into an integral

Now that we know how to find the volume of each shell we need to come up with a way to put that into an integral. The reason for this is that the integral adds up the volume of all of the shells that make up the figure to find the total volume.

In order to do this, we will need to think about the formula for the volume in terms of measurments of the cylinders. We know the three pieces we need to find the volume of one of the shells are the circumference, thickness, and height of the cylinders. Typically when we describe a cylinder, we need two measurements to do this: height and radius. So we want to represent the circumference, thickness, and height in terms of height and radius.

First let’s think about the circumference. We know that the circumference of a circle is always going to be $$circumference=2 \pi r$$ where r is the radius.

The thickness of each shell is a bit strange. As we go from one shell to the next throughout the integral we want to think about what is changing. When we are doing this, we will always want to think about integrating throughout the radius. It’s as if we are starting at the center of the figure and integrating in the direction toward the edge of the figure. So when we go from one shell to the next we travel throughout the radius. Therefore, the thickness of each cylindrical shell with be the distance we travel between each shell. This will just be the change in radius between each shell. Therefore we’ll say the the thickness is $$thickness = dr.$$

Lastly, the height will still be described as the height. We don’t really need to do anything here.

Therefore, using these three conversions we know that the volume of the whole figure can be found with the following integral. $$\int 2 \pi r h \ dr$$

#### Relating it back to our figure

So we have a general outline to set up our integral, but now we need to figure out how our figure fits into these pieces. The easiest way to do this is draw it out with one of the cylindrical shells that makes up our 3-D figure.

So remember earlier I said that when we use this method to find the volume, we are integrating in the direction of the radius of the cylinders. We can see in our drawing that if we start in the center of our cylinder and move toward the edge, we would be going in the x direction. Therefore, we need to think about how to represent our integral in terms of x so we can integrate with respect to x.

So we have 3 pieces of our integral that we need to put in terms of x: r, h, and dr.

#### Finding r

You can see in the drawing above I drew an example of one of the cylindrical shells within our figure. There is also a smaller version of this shell in the upper left hand corner which has a few points labeled which will lie on the various functions that created our bounded region.

We can see the radius of our cylinder would be the distance between its center and edge, which is the distance between the two points labeled $$(0, \ y)$$ and $$(x, \ y)$$. The x-coordinate of that first point will always be 0 because that point lies on the y-axis. And $$(x, y)$$ is some point that lies on the function $$y=x^2-2x+2$$. Since these points have the same y value, the distance between them will just be the distance between their x values. So $$r=x-0$$ $$r=x.$$

#### Finding h

Looking at the labeled cylindrical shell in the drawing above, we can see that the height of the cylinder will be the distance between the points labeled $$(x, \ y)$$ and $$(x, \ 0)$$. Again, $$(x, \ y)$$ is some point on the function $$y=x^2-2x+2$$. And the y-coordinate of that second point will always be 0 because it sits on the x-axis.

It is also important to notice that these two points will sit on the top and bottom edges of every single shell that makes up this figure. This will be true for the inner most shell, the outer most shell, and every shell between them.

Since these these two points have the same x value, the distance between them will simply be the distance between their y values. So $$h=y-0$$ $$h=y$$

But remember we need everything in terms of x, not y. So we need to think about how to represent this height using x instead. Since our y is just the y-coordinate of some point that lies on the function $$y=x^2-2x+2$$, we can replace the y with $$x^2-2x+2$$. So $$h=x^2-2x+2.$$

#### Finding dr

This is actually the simplest part to find. The dr represents the change in the cylinder’s radius as we go from each shell to the next. Since we move in the same direction of the radius as we integrate to find our volume, the change in r should be the same as the change in x between each step. Therefore, we can say that $$dr=dx.$$

#### Putting it all back into an integral

We know that the volume of our figure can be found by using the integral $$\int 2 \pi r h \ dr.$$ And we just found how to represent all of these pieces in terms of x by $$r=x$$ $$h=x^2-2x+2$$ $$dr=dx.$$ So we can substitute all of these pieces into the integral and get something in terms of x that will tell us exactly how to find the volume of our figure. $$V=\int 2 \pi x \big( x^2-2x+2 \big) \ dx$$

But there is actually one more thing we need to consider. Our integral needs bounds. Since we are integrating with respect to x we need to figure out all the x values we want to consider when finding our volume.

To do this we just need to look at the original 2-D region we had bounded by all of our functions. Looking back at our drawings we can see that the entire region goes from $$x=1$$ to $$x=2$$. Therefore, these will be our bounds, telling us that $$V=\int_1^2 2 \pi x \big( x^2-2x+2 \big) \ dx.$$

### 4. Solve the integral

Now that we got our integral set up, all we need to go is evaluate the integral and find the volume it represents.

Before doing that I will simplify things a bit by pulling out the constant $$2 \pi$$ and then simplifying the function by distributing.

$$V=\int_1^2 2 \pi x \big( x^2-2x+2 \big) \ dx$$ $$V=2 \pi \int_1^2 x^3-2x^2+2x \ dx$$ $$V=2 \pi \Bigg[ \frac{1}{4}x^4 – \frac{2}{3}x^3+ x^2 \Bigg]_1^2$$ $$V=2 \pi \Bigg[ \bigg( \frac{1}{4}(2)^4 – \frac{2}{3}(2)^3+ (2)^2 \bigg) \ – \ \bigg( \frac{1}{4}(1)^4 – \frac{2}{3}(1)^3+ (1)^2 \bigg) \Bigg]$$ $$V=2 \pi \Bigg[ \bigg( 4 – \frac{16}{3}+ 4 \bigg) \ – \ \bigg( \frac{1}{4} – \frac{2}{3} + 1 \bigg) \Bigg]$$ $$V=2 \pi \bigg[ \frac{8}{3} – \frac{7}{12} \bigg]$$ $$V=2 \pi \bigg[ \frac{25}{12} \bigg]$$ $$V= \frac{25 \pi}{6}$$

And that’s it! The volume of our 3-D figure is $$\frac{25 \pi}{6}$$ cubic units.

I do quickly want to circle back to a comment I made a while ago. We could have found this volume using the washer method. However, we would have had to split it into two separate integrals to do so. The reason for this is that the inner radius of the washers on the lower half of the figure is formed by $$x=1$$. But the inner radius of the washers on the upper half of our figure is formed by $$y=x^2-2x+2$$. So we’d have to set up one integral for the lower half and another for the upper half, then add the resulting volumes to find the total volume of our figure.

Clearly using the cylindrical shell method is much easier in this case.

If you want more practice on finding volumes of rotation using the shell method, you can find another example here.

Hopefully all of this helps you gain a bit of a better understanding of this method, but as always I’d love to hear your questions if you have any. Just email me at jakesmathlessons@gmail.com and I’ll see if I can help provide a bit more clarification. You can also use the form below to subscribe to my email list and I’ll send you my FREE calc 1 study guide! Just put in your name and email address and I’ll be sure to let you know when I post new content! Feel free to go check out my other lessons and solutions about integrals as well.

## Washer Method Practice Problem

Once you have the disk method down, the next step would be to find the volume of a solid using the washer method. The washer method for finding the volume of a solid is very similar to the disk method with one small added complexity.

You can think of the main difference between these two methods being that the washer method deals with a solid with a piece of it taken out. Exactly as you would expect from the name, a washer is just a disk with a hole taken out of its center. So let’s jump into an example and I’ll explain the difference as we go.

## Example

Find the volume obtained by rotating the area bounded by $$y=x$$ and $$y= \sqrt{x}$$ about the line $$y=1$$.

## Solution

Like I said, finding the volume of this solid is going to be very similar to finding the volume of a solid using the disk method. Therefore, I’m going to follow the same general process as I did when using the disk method.

### 1. Graph the 2-D functions

Just like before, I’ll do this using Wolfram Alpha. Below you can see the graph of $$y=x$$ and $$y=\sqrt{x}$$ and you can see the area that is bounded by these two functions.

### 2. Rotate the 2-D area around the given axis

Now that we have graphed our functions in the 2-D space, we will need to rotate them about our axis of rotation to create a 3-D figure. This is very similar to rotating a single function when using the disk method. The only difference is that the resulting figure will be round figure with something missing out of its center.

Remember, we need to rotate this area between the two functions around the line $$y=1$$, which is above our functions this time. Again, with Wolfram Alpha, we can see what this figure would look like.

### 3. Setting up the integral

This is where things get a little different as a result of using the washer method. We still want to think about taking infinitely thin cylinders from this figure and measuring their volumes.

But let’s think about what these infinitely thin cylinders look like.

Since our figure is a cone-like shape with a piece missing from the middle, our cylinders are also going to have a piece missing from the middle. This makes them look like washers, which is why we call it the washer method! Since the slices of our figure look like washers we can’t find their volume by just finding the volume of a cylinder. There will be something a little extra.

#### What’s different about a washer?

Let’s take a look at the washer below and consider how we might find its volume.

Clearly this figure takes on the shape of a cylinder. If we knew its radius and its height, we could use the formula for the volume of a cylinder to find its volume. However, there is a piece missing.

If we think about the piece that is missing from the cylinder for a moment you can see that this hole also has a cylindrical shape. In other words, we are starting with a cylinder and taking out a smaller cylinder from the middle of it.

So let’s say the distance from the center of these cylinders to the outer edge of this figure is the large radius, R. And we will say that the distance from the center of this figure to the inner edge is the small radius, r. We can think of r as the radius of the small cylinder that was taken out of the large cylinder.

Clearly both of these cylinders would have the same height, which we will call h, because this washer has the same thickness everywhere. So we can label all of these, giving us something like this.

Then we could use the volume of a cylinder formula to say that the volume of the large cylinder before the middle is taken out is $$\pi R^2 h.$$ And we can also say that the volume of the middle piece that’s taken out is $$\pi r^2 h.$$ So if we started with the volume of the large cylinder and take away the volume of the small cylinder, that means the volume of the washer is $$V= \pi R^2 h \ – \ \pi r^2 h.$$

Then we can simplify this a bit by factoring out like terms to get $$V = \pi h \big( R^2 – r^2 \big)$$

#### But what does this have to do with an integral?

Just like we did when we used the disk method, we will need to add up the volume of all of the washers. This is what the integral accomplishes. In order to write our volume as an integral, we first need to come up with the function we will integrate.

Consider the following drawing of the area described in this problem rotated around $$y=1$$ with one of the washer slices depicted.

The green washer in the figure represents one of the infinitely thin washers that we are slicing the figure into. Since we are trying to make this integral represent the sum of all of these washers, we need to think about the volume of each washer in particular.

You can imagine if we were to look at a different washer to the left or right of this one, the inner and outer radius would be different. As we change the x value where the washer sits, both radii would need to change. The inner radius would always reach to the part of this figure resulting from rotating $$y=\sqrt{x}$$ around $$y=1$$. And the outer radius would always reach out to the part of this figure resulting from rotating $$y=x$$ around $$y=1$$.

We already figured out that the volume of a washer would be $$V = \pi h \big( R^2 – r^2 \big).$$ But now we need to apply this to this specific washer in the drawing above.

Let’s look up at the green washer in the above drawing. In order to find its volume, we will need its inner radius, outer radius, and height (or thickness).

The inner radius of this washer would be the distance from the center of the washer to the inner edge. In the drawing above, we can see that this would be the distance between the axis of rotation (the green line labeled $$y=1$$) and the the function which is closer to this axis (the red function labeled $$y=\sqrt{x}$$).

Off to the side of these functions you can see a closer look at this washer with three important points labeled. In this larger version the inner radius is the distance between the point $$(x, \ 1)$$ and $$(x, \ y_1)$$. We know that the center of our disk will always have a y-coordinate of 1 because we rotated our function around the line $$\mathbf{y=1}$$. And we will leave the x-coordinate as the variable x because we are trying to find the volume of any washer along this figure with all different x values, not just the one disk drawn above.

Let’s take a second to think about the other point I mentioned. This point is labeled $$(x, \ y_1)$$. This is just meant to be any (x, y) combination that sits on our function $$\mathbf{y=\sqrt{x}}$$. This point will always have the same x-value as our other labeled point, so the distance between these two points will simply be the distance between their y-coordinates, which are $$y_1$$ and 1. To find the distance between these two values, we just need to do the larger value minus the smaller one. Looking at the drawing, you can see that $$y=1$$ is above $$y=\sqrt{x}$$ over the entire domain of the area we care about. Therefore, the larger y value minus the smaller one, is $$1-y_1.$$

But we need this to be in terms of x. Remember we realized earlier that the inner radius of our washer will depend on x, so we want everything in terms of x. We know that our point $$(x, \ y_1)$$ is some point that lies on the function $$y=\sqrt{x}$$. So we know $$\mathbf{y_1=\sqrt{x}}$$. Therefore, if the inner radius of our washer is $$1-y_1$$, we can also say that the radius is $$1-\sqrt{x}$$. So the inner radius of this washer will be $$r=1-\sqrt{x}.$$

We could plug in any x value into this to find the inner radius of the washer that corresponds with that specific x value. So when we integrate across a range of x values, we will be taking into account the inner radius of the washers with all of the x values in that range. But let’s not get ahead of ourselves. We have a bit more work to do before we can do that.

We will find the outer radius very similarly to how we found the inner radius. The outer radius of this washer would be the distance from the center of the washer to the outer edge. In the drawing above, we can see that this would be the distance between the axis of rotation (the green line labeled $$y=1$$) and the the function which is farther from this axis (the blue function labeled $$y=x$$).

Again, let’s take a look at the washer drawn off to the side of our figure. This time we are looking for the outer radius. The outer radius would be shown here as the distance between the points $$(x, \ 1)$$ and $$(x, \ y_2)$$.

Since these two points have the same x value, the distance between them will be the same as the distance between their y values. To find this we just need to take the larger one and subtract the smaller of the two. The point $$(x, \ 1)$$ will always lie on $$y=1$$ and the point $$(x, \ y_2)$$ will always lie on $$y=x$$. Since $$y=1$$ is above $$y=x$$ between $$x=0$$ and $$x=1$$, we know that 1 will be larger than $$y_2$$. Therefore, the distance between these two points is $$1-y_2.$$

But again, we need this radius to be written in terms of x. We know that our point $$(x, \ y_2)$$ is some point that lies on the function $$y=x$$. So we know $$\mathbf{y_2=x}$$. Therefore, if the outer radius of our washer is $$1-y_2$$, we can also say that the radius is $$1-x$$. So the outer radius of this washer will be $$R=1-x.$$

#### Finding the height (or thickness)

The height of our infinitely thin cylinders or washers is actually quite simple. Just like when we integrate a 2-D function to find the area under the curve, our slices here are all the same width. We don’t have to worry about each washer, or cylinder, having a different height.

The height of each cylinder will just be how far we always move over before taking another slice. Since we are moving over in the x direction as we imagine the next slice, this can simply be our change in x between the slices. Change in x is always represented as dx. So we can simply say the height of each cylinder is $$h=dx.$$

#### Back to the integral

Like I said before, all the integral will do is go through all the x values in our domain and add up the volumes of all of the infinitely thin washers. In order for it to achieve this, we need to put a function for the volume of each washer that depends on x. We already know that the volume of a washer in general would be $$V = \pi h \big( R^2 – r^2 \big).$$

This means that our integral might look something like this $$\int \pi h \big( R^2 – r^2 \big).$$

But this doesn’t really have any meaning on its own. In order to give this meaning we need to represent this volume in terms of x and give the integral a domain of x values to integrate over.

Remember we also found the inner radius, outer radius, and height of the washers that make up our figure to be $$r=1-\sqrt{x},$$ $$R=1-x,$$ $$h=dx.$$

Putting all these into our integral, along with the fact that our figure takes on all x values between $$x=0$$ and $$x=1$$ tells us that the volume of this 3-D figure is $$V = \int_0^1 \pi (dx) \Big( \big( 1-x \big)^2 – \big( 1-\sqrt{x} \big)^2 \Big).$$

But we should rewrite this in a form that is more in line with how integrals are usually formatted. And we can also pull our constant $$\pi$$ outside of the integral. $$V = \pi \int_0^1 \big( 1-x \big)^2 – \big( 1-\sqrt{x} \big)^2 \ dx$$

### 4. Solve the integral

We made it through the hard part! Now that we created our integral to represent the volume, we just need to evaluate the integral. Before we integrate this, let’s start with simplifying it.

In order to simplify this function that we need to integrate, the first step would be to F.O.I.L. out each portion. Remember $$(1-x)^2$$ is NOT $$(1^2-x^2)$$. Instead we need to treat $$(1-x)^2$$ as if it were $$(1-x)(1-x)$$.

$$V = \pi \int_0^1 \big( 1-x \big)^2 – \big( 1-\sqrt{x} \big)^2 \ dx$$ $$V = \pi \int_0^1 (1-x)(1-x) – \big(1-\sqrt{x})(1-\sqrt{x} \big) \ dx$$ $$V = \pi \int_0^1 \big(1-2x+x^2 \big) – \big(1-2\sqrt{x}+x \big) \ dx$$ $$V = \pi \int_0^1 1-2x+x^2 – 1+2\sqrt{x}-x \ dx$$ $$V = \pi \int_0^1 x^2 -3x +2\sqrt{x} \ dx$$ $$V = \pi \int_0^1 x^2 -3x +2x^{\frac{1}{2}} \ dx$$

Now we have a function that is fairly simple to integrate. All we need to do to integrate this is use the power rule.

$$V = \pi \Bigg[ \frac{1}{3}x^3 – \frac{3}{2}x^2 + \frac{4}{3}x^{\frac{3}{2}} \Bigg]_0^1$$ $$V = \pi \Bigg[ \bigg( \frac{1}{3}1^3 – \frac{3}{2}1^2 + \frac{4}{3}1^{\frac{3}{2}}\bigg) – \bigg( \frac{1}{3}0^3 – \frac{3}{2}0^2 + \frac{4}{3}0^{\frac{3}{2}} \bigg) \Bigg]$$ $$V = \pi \bigg( \frac{1}{3} – \frac{3}{2} + \frac{4}{3} \bigg)$$ $$V = \pi \bigg( \frac{2}{6} – \frac{9}{6} + \frac{8}{6} \bigg)$$ $$V = \frac{1}{6} \pi = \frac{\pi}{6}$$

So we know the volume of this solid is $$\frac{\pi}{6}$$! Hopefully this has helped you with the washer method, but if there’s still a topic you’d like to learn about take a look at some of my other lessons and problem solutions about integrals. You can also get some more practice with the washer method here. If you can’t find the topic or question you’re looking for just let me know by emailing me at jakesmathlessons@gmail.com!

I also encourage you to join my email list! Just enter your name and email below and I’ll send you my calc 1 study guide as a FREE welcome gift!

## RELATED RATES – Cylinder Problem

A cylindrical tank with radius 5 m is being ﬁlled with water at a rate of 3 $$\frac{m^3}{min}$$. How fast is the height of the water increasing?

This is an interesting example because at first glance it doesn’t seem like we have been given enough information to solve this problem. If you compare this to the related rates cone problem we did, you can notice a few things that were given in that example but not this one.

• We don’t know the height of the cylindrical tank.
• We don’t know the height of the water at the instant we need to find its rate of change.

These things were both given in the cone problem we did.

You may want to check out this video of this problem. I solved it in a slightly different way in the video than I did in this post. Check out both so you can see it a couple different ways and decide which one makes more sense to you!

#### How do we deal with this missing information?

We actually don’t need it! Because the surface of the water in this tank will always be a circle with radius 5 m as this tank fills up, the height of the water or the tank won’t impact our answer. So there’s two possible ways to deal with this:

• Create an equation that doesn’t include the height of the tank or the water,
• or plug in some random value for the height and see what we get. If this one works we should be able to plug in any height and get the same answer for all of them.

One of these options actually won’t work, but we will explore them further later in this example. Let’s not get ahead of ourselves. Since we have a related rates problem here, we will want to follow the same four steps as all other related rates problems.

## 1. Draw a sketch

As with any related rates problem, the first thing we need to do is draw the situation being described to us. This is a relatively simple situation being described, so we can go ahead and draw it.

Looking at the above drawing, you can see that water is being poured into a cylindrical tank at a rate of 3 $$\frac{m^3}{min}$$. The height of the tank is unknown, but we know the radius is 5 m. We can also see that the water level is rising, but we don’t know the height of the water at this instant.

## 2. Come up with your equation

Now that we have a drawing of the situation being described, we need to come up with our equation. To do this, we need to sort out what information we know and what we are looking for.

#### What are we looking for?

The question is asking us to find how fast the height of the water is increasing. Once we take the derivative of our equation, that will introduce the variable representing “how fast things are changing.” Therefore, we know that we will need the height of the water to be in our equation.

Remember in the last section I said we may want to create an equation that didn’t contain the height of either cylinder because we have no information about them? Well now we know that’s not really an option. The question is asking us to find the rate of change of the height of one of the cylinders. To find this, we need the height of the cylinder to be in the equation.

#### What do we know about?

This question didn’t give us a lot of information but we can figure out a little extra. First let’s think about what was directly given. In order to do this, we should consider that this example essentially has two cylinders in it. The large cylinder is the tank, and the small cylinder is the water in the tank.

• We know that water is flowing into the tank at a rate of 3 $$\frac{m^3}{min}$$. This means that the volume of the small cone is increasing at a rate of 3 $$\mathbf{\frac{m^3}{min}}$$.
• The problem also says that the tank has a radius of 5 m.

And this is all the information that is explicitly given in the problem. However, there is one other fact we can infer. As the tank is filled with water, the liquid takes on a cylindrical shape as well. The cylinder made up of the water would be shorter than the tank, but it would have the same width. This leads to our last fact.

• We can conclude that the water also has a radius of 5 m.

#### Putting it into an equation

So far in this section, we have figured out that we will need to include the height of the water. We also determined that the information we know is about the radius of both cylinders and the rate of change of the volume of the small cylinder.

So we need our equation to relate the height, radius, and volume of the liquid. Because of this, I think a good place to start would be with the equation for the volume of a cylinder:

$$V=\pi r^2 h$$

In this equation V is the volume, r is the radius, and h is the height of the liquid in the tank.

One thing worth pointing out is that once we take the derivative of this equation, we will still have an h. But we don’t know anything about the height of the liquid in the tank at this instant. It seems like we wouldn’t have enough information to successfully use this equation, but it’s possible that the height at a given moment won’t actually impact the rate of change of the height. If this is true, we should be able to use this equation anyway. So let’s proceed and see what happens.

## 3. Implicit differentiation

As with any related rates problem, once we create our equation we need to take its derivative. Since we are taking the derivative with respect to time, we need to treat V, r, and h as functions of time rather than variables. Therefore, we need to use the chain rule. In this case, since we have r and h being multiplied, we will also have to use the product rule.

$$\frac{d}{dt}[V]=\frac{d}{dt} \big[\pi r^2 h \big]$$

### Using the product rule

To find the derivative of the right side of this equation we need to start by using the product rule. So we are trying to find

$$\frac{d}{dt} \big[\pi r^2 h \big].$$

Since $$\pi$$ is a constant being multiplied by the rest of the function we are taking the derivative of, we can pull it out of the derivative and deal with it later. Therefore, we can think of the right side of our equation as

$$\pi \frac{d}{dt} \big[r^2 h \big].$$

So we need to find the derivative of $$r^2h$$. Just as I did in the product rule lesson, we will start by deciding which part of this equation we will call f and g.

#### Choosing f and g

It doesn’t really make a difference which piece of this function we call f and which we call g. As long as we make a decision now and stick with it throughout our solution, it will work out in the end. We will say

$$f=r^2,$$

$$g=h.$$

#### Finding f’ and g’

Now that we have figured out f and g, the next step of the product rule is to find each of their derivatives. Keep in mind that we are taking the derivative with respect to time in this example.

To find f’, we will need to use the chain rule as well since r is a function of time. doing this tells us that

$$f’=2r \frac{dr}{dt}$$

Lastly, we need to find g’ by taking the derivative of g with respect to time.

$$g’=\frac{dh}{dt}$$

#### Putting the pieces of the product rule together

Now we have figured out all four of the pieces of the product rule, so we can just plug them into the product rule formula to find the derivative we’re looking for.

$$\frac{d}{dt} \big[r^2 h \big] = 2r \frac{dr}{dt} \cdot h + \frac{dh}{dt} \cdot r^2$$

And lastly, we just need to bring the $$\pi$$ back into the equation.

$$\frac{d}{dt} \big[\pi r^2 h \big] = \pi \bigg[ 2r \frac{dr}{dt} \cdot h + \frac{dh}{dt} \cdot r^2 \bigg]$$

### Putting it all together

Now that we have the derivative of the right side of our equation, we can go back and figure out the left side.

$$\frac{dV}{dt}= \pi \bigg[ 2r \frac{dr}{dt}h + \frac{dh}{dt}r^2 \bigg]$$

## 4. Solve for desired rate of change

The last step of any related rates problem is to solve for the rate of change we need to find. The question asks us to find how fast the height of the water is increasing. This is exactly what $$\frac{dh}{dt}$$ represents, so we need to isolate that variable.

$$\frac{dV}{dt}= \pi \bigg[ 2r \frac{dr}{dt}h + \frac{dh}{dt}r^2 \bigg]$$

$$\frac{1}{\pi}\frac{dV}{dt}= 2r \frac{dr}{dt}h + \frac{dh}{dt}r^2$$

$$\frac{1}{\pi}\frac{dV}{dt} – 2r \frac{dr}{dt}h = \frac{dh}{dt}r^2$$

$$\frac{1}{r^2} \bigg[ \frac{1}{\pi}\frac{dV}{dt} – 2r \frac{dr}{dt}h \bigg] = \frac{dh}{dt}$$

Now that we have isolated the term we need to solve for, we just need to plug in the values for all of the other variables. Before we just into this, I want to focus on just one of these variables.

#### What do we do with h?

Remember back in the beginning of this solution I said we may need to make up some value for h and just go with it? Let’s consider this for a moment.

Notice that h only appears in our equation in one place and it is being multiplied by two other variables, r and $$\frac{dr}{dt}$$. Think about what $$\frac{dr}{dt}$$ represents. It is the rate of change of the radius of the water. But our water has a constant radius, it’s always 5 m.

Since the radius of the cylinder is never changing, its rate of change must always be zero! Therefore, we know that

$$\frac{dr}{dt} = 0.$$

Since h is being multiplied by another term that is always zero, it’s not going to matter what h is. We can essentially use any number for h and it won’t matter because we are going to multiply it by zero anyways. We can also just leave it as h and not plug anything in for it.

#### Back to plugging in our values

We have already figured out that $$\frac{dr}{dt}=0$$ and luckily the other variables’ values were given.

Looking back at the original sketch, you can see that the radius of our cylinder is 5 m. So

$$r=5.$$

Also, you can see that water is flowing into the tank at a rate of 3 $$\frac{m^3}{min}$$. Since this is the only factor that will be impacting the volume of water in the tank, this must be the exact rate that the volume of the water is increasing.

$$\frac{dV}{dt}=3$$

Now we can plug all of these into the equation then simplifying to get our answer!

$$\frac{1}{r^2} \bigg[ \frac{1}{\pi}\frac{dV}{dt} – 2r \frac{dr}{dt}h \bigg] = \frac{dh}{dt}$$

$$\frac{1}{(5)^2} \bigg[ \frac{1}{\pi}(3) – 2(5)(0)h \bigg] = \frac{dh}{dt}$$

$$\frac{1}{25} \bigg[ \frac{3}{\pi} – 0 \bigg] = \frac{dh}{dt}$$

$$\frac{1}{25} \cdot \frac{3}{\pi} = \frac{dh}{dt}$$

$$\frac{3}{25 \pi} = \frac{dh}{dt}$$

So we now know that the rate of change of the height of the water is $$\frac{3}{25 \pi}$$ no matter what the height of the cylinder is at that moment. In other words, we can say that the height of the water is increasing at a rate of

$$\mathbf{\frac{3}{25 \pi} \ \frac{m}{min}}.$$

If you’re still having some trouble with related rates problems or just want some more practice you should check out my related rates lesson. At the bottom of this lesson there is a list of related rates problems that I have posted a solution of. I also have several other lessons and problems on the derivatives page you can check out. If you can’t find the topic or question you’re looking for just let me know by emailing me at jakesmathlessons@gmail.com!

Also, if you want a copy of my calculus 1 study guide, just enter your name and email below and I’ll send you a copy and add you to my email list so you can see as soon as I have more knowledge to share!