6. Applications of Integration

# 6.4 Arc Length of a Curve and Surface Area

### Learning Objectives

• Determine the length of a curve, between two points.
• Determine the length of a curve, between two points.
• Find the surface area of a solid of revolution.

In this section, we use definite integrals to find the arc length of a curve. We can think of arc length as the distance you would travel if you were walking along the path of the curve. Many real-world applications involve arc length. If a rocket is launched along a parabolic path, we might want to know how far the rocket travels. Or, if a curve on a map represents a road, we might want to know how far we have to drive to reach our destination.

We begin by calculating the arc length of curves defined as functions of then we examine the same process for curves defined as functions of (The process is identical, with the roles of and reversed.) The techniques we use to find arc length can be extended to find the surface area of a surface of revolution, and we close the section with an examination of this concept.

# Arc Length of the Curve = ()

In previous applications of integration, we required the function to be integrable, or at most continuous. However, for calculating arc length we have a more stringent requirement for Here, we require to be differentiable, and furthermore we require its derivative, to be continuous. Functions like this, which have continuous derivatives, are called smooth. (This property comes up again in later chapters.)

Let be a smooth function defined over We want to calculate the length of the curve from the point to the point We start by using line segments to approximate the length of the curve. For let be a regular partition of Then, for construct a line segment from the point to the point Although it might seem logical to use either horizontal or vertical line segments, we want our line segments to approximate the curve as closely as possible. (Figure) depicts this construct for

We can approximate the length of a curve by adding line segments.

To help us find the length of each line segment, we look at the change in vertical distance as well as the change in horizontal distance over each interval. Because we have used a regular partition, the change in horizontal distance over each interval is given by The change in vertical distance varies from interval to interval, though, so we use to represent the change in vertical distance over the interval as shown in (Figure). Note that some (or all) may be negative.

A representative line segment approximates the curve over the interval

By the Pythagorean theorem, the length of the line segment is We can also write this as Now, by the Mean Value Theorem, there is a point such that Then the length of the line segment is given by Adding up the lengths of all the line segments, we get

This is a Riemann sum. Taking the limit as we have

We summarize these findings in the following theorem.

### Arc Length for = ()

Let be a smooth function over the interval Then the arc length of the portion of the graph of from the point to the point is given by

Note that we are integrating an expression involving so we need to be sure is integrable. This is why we require to be smooth. The following example shows how to apply the theorem.

### Calculating the Arc Length of a Function of

Let Calculate the arc length of the graph of over the interval Round the answer to three decimal places.

#### Solution

We have so Then, the arc length is

Substitute Then, When then and when then Thus,

Let Calculate the arc length of the graph of over the interval Round the answer to three decimal places.

#### Hint

Use the process from the previous example. Don’t forget to change the limits of integration.

Although it is nice to have a formula for calculating arc length, this particular theorem can generate expressions that are difficult to integrate. We study some techniques for integration in Introduction to Techniques of Integration in the second volume of this text. In some cases, we may have to use a computer or calculator to approximate the value of the integral.

### Using a Computer or Calculator to Determine the Arc Length of a Function of

Let Calculate the arc length of the graph of over the interval

#### Solution

We have so Then the arc length is given by

Using a computer to approximate the value of this integral, we get

Let Calculate the arc length of the graph of over the interval Use a computer or calculator to approximate the value of the integral.

#### Hint

Use the process from the previous example.

# Arc Length of the Curve = ()

We have just seen how to approximate the length of a curve with line segments. If we want to find the arc length of the graph of a function of we can repeat the same process, except we partition the instead of the (Figure) shows a representative line segment.

A representative line segment over the interval

Then the length of the line segment is which can also be written as If we now follow the same development we did earlier, we get a formula for arc length of a function

### Arc Length for = ()

Let be a smooth function over an interval Then, the arc length of the graph of from the point to the point is given by

### Calculating the Arc Length of a Function of

Let Calculate the arc length of the graph of over the interval

#### Solution

We have so Then the arc length is

Using a computer to approximate the value of this integral, we obtain

Let Calculate the arc length of the graph of over the interval Use a computer or calculator to approximate the value of the integral.

#### Hint

Use the process from the previous example.

# Area of a Surface of Revolution

The concepts we used to find the arc length of a curve can be extended to find the surface area of a surface of revolution. Surface area is the total area of the outer layer of an object. For objects such as cubes or bricks, the surface area of the object is the sum of the areas of all of its faces. For curved surfaces, the situation is a little more complex. Let be a nonnegative smooth function over the interval We wish to find the surface area of the surface of revolution created by revolving the graph of around the as shown in the following figure.

(a) A curve representing the function (b) The surface of revolution formed by revolving the graph of around the

As we have done many times before, we are going to partition the interval and approximate the surface area by calculating the surface area of simpler shapes. We start by using line segments to approximate the curve, as we did earlier in this section. For let be a regular partition of Then, for construct a line segment from the point to the point Now, revolve these line segments around the to generate an approximation of the surface of revolution as shown in the following figure.

(a) Approximating with line segments. (b) The surface of revolution formed by revolving the line segments around the

Notice that when each line segment is revolved around the axis, it produces a band. These bands are actually pieces of cones (think of an ice cream cone with the pointy end cut off). A piece of a cone like this is called a frustum of a cone.

To find the surface area of the band, we need to find the lateral surface area, of the frustum (the area of just the slanted outside surface of the frustum, not including the areas of the top or bottom faces). Let and be the radii of the wide end and the narrow end of the frustum, respectively, and let be the slant height of the frustum as shown in the following figure.

A frustum of a cone can approximate a small part of surface area.

We know the lateral surface area of a cone is given by

where is the radius of the base of the cone and is the slant height (see the following figure).

The lateral surface area of the cone is given by

Since a frustum can be thought of as a piece of a cone, the lateral surface area of the frustum is given by the lateral surface area of the whole cone less the lateral surface area of the smaller cone (the pointy tip) that was cut off (see the following figure).

Calculating the lateral surface area of a frustum of a cone.

The cross-sections of the small cone and the large cone are similar triangles, so we see that

Solving for we get

Then the lateral surface area (SA) of the frustum is

Let’s now use this formula to calculate the surface area of each of the bands formed by revolving the line segments around the A representative band is shown in the following figure.

A representative band used for determining surface area.

Note that the slant height of this frustum is just the length of the line segment used to generate it. So, applying the surface area formula, we have

Now, as we did in the development of the arc length formula, we apply the Mean Value Theorem to select such that This gives us

Furthermore, since is continuous, by the Intermediate Value Theorem, there is a point such that so we get

Then the approximate surface area of the whole surface of revolution is given by

This almost looks like a Riemann sum, except we have functions evaluated at two different points, and over the interval Although we do not examine the details here, it turns out that because is smooth, if we let the limit works the same as a Riemann sum even with the two different evaluation points. This makes sense intuitively. Both and are in the interval so it makes sense that as both and approach Those of you who are interested in the details should consult an advanced calculus text.

Taking the limit as we get

As with arc length, we can conduct a similar development for functions of to get a formula for the surface area of surfaces of revolution about the These findings are summarized in the following theorem.

### Surface Area of a Surface of Revolution

Let be a nonnegative smooth function over the interval Then, the surface area of the surface of revolution formed by revolving the graph of around the -axis is given by

Similarly, let be a nonnegative smooth function over the interval Then, the surface area of the surface of revolution formed by revolving the graph of around the is given by

### Calculating the Surface Area of a Surface of Revolution 1

Let over the interval Find the surface area of the surface generated by revolving the graph of around the Round the answer to three decimal places.

#### Solution

The graph of and the surface of rotation are shown in the following figure.

(a) The graph of (b) The surface of revolution.

We have Then, and Then,

Let Then, When and when This gives us

Let over the interval Find the surface area of the surface generated by revolving the graph of around the Round the answer to three decimal places.

#### Hint

Use the process from the previous example.

### Calculating the Surface Area of a Surface of Revolution 2

Let Consider the portion of the curve where Find the surface area of the surface generated by revolving the graph of around the

#### Solution

Notice that we are revolving the curve around the and the interval is in terms of so we want to rewrite the function as a function of . We get The graph of and the surface of rotation are shown in the following figure.

(a) The graph of (b) The surface of revolution.

We have so and Then

Let Then When and when Then

Let over the interval Find the surface area of the surface generated by revolving the graph of around the

#### Hint

Use the process from the previous example.

### Key Concepts

• The arc length of a curve can be calculated using a definite integral.
• The arc length is first approximated using line segments, which generates a Riemann sum. Taking a limit then gives us the definite integral formula. The same process can be applied to functions of
• The concepts used to calculate the arc length can be generalized to find the surface area of a surface of revolution.
• The integrals generated by both the arc length and surface area formulas are often difficult to evaluate. It may be necessary to use a computer or calculator to approximate the values of the integrals.

# Key Equations

• Arc Length of a Function of
• Arc Length of a Function of
• Surface Area of a Function of

For the following exercises, find the length of the functions over the given interval.

1.

2.

3.

#### Solution

4. Pick an arbitrary linear function over any interval of your choice Determine the length of the function and then prove the length is correct by using geometry.

5. Find the surface area of the volume generated when the curve revolves around the from to as seen here.

#### Solution

6. Find the surface area of the volume generated when the curve revolves around the from to

For the following exercises, find the lengths of the functions of over the given interval. If you cannot evaluate the integral exactly, use technology to approximate it.

7. from

8.  from

9.  from

10.  from to

11. [T] on to

2.0035

12.  from

13.  from

14.  from

15.  from

#### Solution

10

16. [T] on

For the following exercises, find the lengths of the functions of over the given interval. If you cannot evaluate the integral exactly, use technology to approximate it.

17.  from to

18. from

19.  from to

20. [T] from to

21. from

22. from to

23. [T] from to

1.201

24. [T] from to

25. [T] from

#### Solution

15.2341

26. [T] on to

For the following exercises, find the surface area of the volume generated when the following curves revolve around the If you cannot evaluate the integral exactly, use your calculator to approximate it.

27.  from to

28.  from to

29.  from

30. [T] from

31. from

32.  from

33.  from

#### Solution

34. [T] from

For the following exercises, find the surface area of the volume generated when the following curves revolve around the If you cannot evaluate the integral exactly, use your calculator to approximate it.

35. from

36. from

37.  from

38. [T] from to

39.  from

40. [T] from to

41. [T] from to

#### Solution

25.645

42. [T] from to

43. The base of a lamp is constructed by revolving a quarter circle around the from to as seen here. Create an integral for the surface area of this curve and compute it.

#### Solution

44. A light bulb is a sphere with radius in. with the bottom sliced off to fit exactly onto a cylinder of radius in. and length in., as seen here. The sphere is cut off at the bottom to fit exactly onto the cylinder, so the radius of the cut is in. Find the surface area (not including the top or bottom of the cylinder).

45. [T] A lampshade is constructed by rotating around the from to as seen here. Determine how much material you would need to construct this lampshade—that is, the surface area—accurate to four decimal places.

#### Solution

10.5017

46. [T] An anchor drags behind a boat according to the function where represents the depth beneath the boat and is the horizontal distance of the anchor from the back of the boat. If the anchor is 23 ft below the boat, how much rope do you have to pull to reach the anchor? Round your answer to three decimal places.

47. [T] You are building a bridge that will span 10 ft. You intend to add decorative rope in the shape of where is the distance in feet from one end of the bridge. Find out how much rope you need to buy, rounded to the nearest foot.

#### Solution

23 ft

For the following exercises, find the exact arc length for the following problems over the given interval.

48.  from to (Hint: Recall trigonometric identities.)

49. Draw graphs of and For as increases, formulate a prediction on the arc length from to Now, compute the lengths of these three functions and determine whether your prediction is correct.

#### Solution

2

50. Compare the lengths of the parabola and the line from as increases. What do you notice?

51. Solve for the length of from Show that from to is twice as long. Graph both functions and explain why this is so.

#### Solution

52. [T] Which is longer between and the hyperbola or the graph of

54. Explain why the surface area is infinite when is rotated around the for but the volume is finite.