FURTHER APPLICATIONS OF INTEGRATION

8 FURTHER

APPLICATIONS OF INTEGRATION

The length of a curve is the limit of lengths of inscribed polygons.

We looked at some applications of integrals in Chapter 6: areas, volumes, work, and average values. Here we explore some of the many other geometric applications of integration—the length of a curve, the area of a surface—as well as quantities of interest in physics, engineering, biology, economics, and statistics. For instance, we will investigate the center of gravity of a plate, the force exerted by water pressure on a dam, the flow of blood from the human heart, and the average time spent on hold during a customer support telephone call.

8.1 ARC LENGTH What do we mean by the length of a curve? We might think of fitting a piece of string to

the curve in Figure 1 and then measuring the string against a ruler. But that might be difficult to do with much accuracy if we have a complicated curve. We need a precise definition for the length of an arc of a curve, in the same spirit as the definitions we devel-

FIGURE 1

oped for the concepts of area and volume.

If the curve is a polygon, we can easily find its length; we just add the lengths of the TEC Visual 8.1 shows an animation

line segments that form the polygon. (We can use the distance formula to find the distance

of Figure 2.

between the endpoints of each segment.) We are going to define the length of a general curve by first approximating it by a polygon and then taking a limit as the number of seg- ments of the polygon is increased. This process is familiar for the case of a circle, where the circumference is the limit of lengths of inscribed polygons (see Figure 2).

Now suppose that a curve is defined by the equation C , where f is continuous and a艋x艋b . We obtain a polygonal approximation to C by dividing the interval into n subintervals with endpoints x 0 ,x 1 ,...,x n and equal width ⌬x . If y i

i , then the point P i i ,y i lies on C and the polygon with vertices P 0 , P 1 ,..., P n , illustrated in

Figure 3, is an approximation to . C

The length L of C is approximately the length of this polygon and the approximation gets better as we let n increase. (See Figure 4, where the arc of the curve between P i⫺ 1 and P i has been magnified and approximations with successively smaller values of ⌬x are shown.) Therefore we define the length L of the curve C with equation

a艋x艋b , as the limit of the lengths of these inscribed polygons (if the limit exists):

Notice that the procedure for defining arc length is very similar to the procedure we

P i-1

used for defining area and volume: We divided the curve into a large number of small parts.

We then found the approximate lengths of the small parts and added them. Finally, we took the limit as nl⬁ .

P i-1

The definition of arc length given by Equation 1 is not very convenient for compu- tational purposes, but we can derive an integral formula for in the case where has a L f continuous derivative. [Such a function is called smooth because a small change in f x

P i-1

produces a small change in f⬘ .]

If we let ⌬y i 苷y i ⫺y i⫺ 1 , then

FIGURE 4

P i⫺ 1 P i i ⫺x i⫺ 1 2 ⫹

i ⫺y i⫺ 1 2 2 ⫹

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION

By applying the Mean Value Theorem to on the interval f i⫺ 1 ,x i , we find that there is a

number between and such x i * x i⫺ 1 x i that

i⫺ 1 i

i ⫺x i⫺ 1

that is,

⌬y i

Thus we have

P i⫺ 1 P i

2 2 i 2 ⌬x

(since ) ⌬x ⬎ 0

Therefore, by Definition 1,

We recognize this expression as being equal to

dx

by the definition of a definite integral. This integral exists because the function

2 is continuous. Thus we have proved the following theorem:

2 THE ARC LENGTH FORMULA If f⬘ is continuous on

, then the length of

the curve

, a艋x艋b , is

L苷 b

y a dx

If we use Leibniz notation for derivatives, we can write the arc length formula as follows:

b dy 2

y a dx

EXAMPLE 1 Find the length of the arc of the semicubical parabola y 2 y 3 苷x between the

points

and

. (See Figure 5.)

SOLUTION For the top half of the curve we have

3 ¥=˛ 1 y苷x 苷

3 dy

dx and so the arc length formula gives

1 y dx 1

If we substitute

u苷1⫹ 13

9 , then

9 dx . When

du 苷 4 x苷1 u苷 4 ; when x苷4 , u 苷 10 .

SECTION 8.1 ARC LENGTH

N As a check on our answer to Example 1, notice

Therefore

from Figure 5 that the arc length ought to be slightly larger than the distance from

9 y du 苷 9 ⴢ 3 u ] 13

, which is

s58 8 [ 3 10 3 ⫺ 13 ( 1 ) ] ( 80 s10 ⫺ 13 s13 ) 苷 27 4 苷 27 M

According to our calculation in Example 1, we have

is continuous, then by inter- L苷 27 1 ( 80 s10 ⫺ 13 s13 ) changing the roles of and in Formula 2 or Equation 3, we obtain the following formula x y

If a curve has the equation

, c艋y艋d , and

Sure enough, this is a bit greater than the length

for its length:

of the line segment.

y c dy

4 2 L苷 dx y

V EXAMPLE 2 Find the length of the arc of the parabola y 2 苷x from to .

SOLUTION Since x苷y 2 , we have dx

, and Formula 4 gives

dy 苷 s1 ⫹ 4y y 2 y dy

0 dy 0

We make the trigonometric substitution

2 ␪ d␪ and s1 ⫹ 4y 2 苷 s1 ⫹ tan 2 ␪ 苷 sec ␪ . ⫺ When , y苷0 tan ␪ 苷 0 , so ; ␪苷0 when , y苷1

1 y苷 1 tan ␪ dy 苷 sec 2

2 , which gives

, so , say. tan ␪ 苷 2 1 ␪ 苷 tan 2苷␣ Thus

L苷 3 y y

0 sec ␪ ⴢ 2 sec ␪ d␪ 苷 2 sec 0 ␪ d␪

2 ⴢ 2 [ sec ␪ tan ␪ ⫹ ln sec ␪ ⫹ tan ␪ 0 (from Example 8 in Section 7.2)

4 ( sec ␣ tan ␣ ⫹ ln sec ␣ ⫹ tan ␣ )

(We could have used Formula 21 in the Table of Integrals.) Since tan ␣ 苷 2 , we have

sec 2 2 ␣ 苷 1 ⫹ tan ␣苷5 , so and sec ␣ 苷 s5

s5 ⫹ ln ( s5 ⫹ 2 )

L苷

L n length is computed in Example 2, together with polygonal approximations having n苷1 and

Figure 6 shows the arc of the parabola whose

1 1.414 n苷2 line segments, respectively. For n苷1

x=¥

2 1.445 the approximate length is L 1 苷 s2 , the diago- 4 1.464

nal of a square. The table shows the approxima- 8 1.472 tions L n that we get by dividing

16 1.476 equal subintervals. Notice that each time we

into n

double the number of sides of the polygon, we 32 1.478

get closer to the exact length, which is 1 64 1.479

s5 ln ( s5 ⫹ 2

L苷 ⫹

2 4 FIGURE 6

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION

Because of the presence of the square root sign in Formulas 2 and 4, the calculation of an arc length often leads to an integral that is very difficult or even impossible to evaluate explicitly. Thus we sometimes have to be content with finding an approximation to the length of a curve, as in the following example.

V EXAMPLE 3

(a) Set up an integral for the length of the arc of the hyperbola xy 苷 1 from the point

to the point

(b) Use Simpson’s Rule with n 苷 10 to estimate the arc length. SOLUTION

(a) We have

and so the arc length is

1 4 2 dx y 1 x y 1 x

(b) Using Simpson’s Rule (see Section 7.7) with a苷1 , b苷2 , n 苷 10 , ⌬x 苷 0.1 , and

f 4 , we have

L苷 dx

N Checking the value of the definite integral with a more accurate approximation produced by

⌬x

a computer algebra system, we see that the

approximation using Simpson’s Rule is accurate to four decimal places.

THE ARC LENGTH FUNCTION We will find it useful to have a function that measures the arc length of a curve from a par-

ticular starting point to any other point on the curve. Thus if a smooth curve C has the

equation

, a艋x艋b , let s

be the distance along C from the initial point

P 0 to the point Q

. Then is a function, called the arc length function, and, s

by Formula 2,

a dt

(We have replaced the variable of integration by so that does not have two meanings.) t x We can use Part 1 of the Fundamental Theorem of Calculus to differentiate Equation 5 (since the integrand is continuous):

dx

6 ds

2 dy

dx

SECTION 8.1 ARC LENGTH

Equation 6 shows that the rate of change of with respect to is always at least 1 and is s x equal to 1 when f⬘ , the slope of the curve, is 0. The differential of arc length is

dx

7 ds 苷 dy 1⫹

dx

and this equation is sometimes written in the symmetric form

y ds

The geometric interpretation of Equation 8 is shown in Figure 7. It can be used as a

dy

mnemonic device for remembering both of the Formulas 3 and 4. If we write L苷 x ds , then from Equation 8 either we can solve to get (7), which gives (3), or we can solve to get

Îs Îy

dx

dy

FIGURE 7 which gives (4).

V EXAMPLE 4 Find the arc length function for the curve y苷x 2 ⫺ 1 8 ln x taking P 0

as the starting point. SOLUTION If , then f 2 ⫺ 1 8 ln x

1 f⬘ 8x

8x 2 64x 2

8x

苷 4x ⫹ ⫹ 64x 2 苷 2 2x ⫹

1 苷 2x ⫹ 8x

Thus the arc length function is given by

dt

1 8t

2t ⫺

dt 苷 t

8 ln t ]

2 ⫹ 苷x 1

8 ln x ⫺ 1

For instance, the arc length along the curve from

to

is

2 ⫹ 1 ln 3

8 ln 3 ⫺ 1 苷 8 ⫹

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION

1 s(x)

N Figure 8 shows the interpretation of the arc

length function in Example 4. Figure 9 shows the

0 x graph of this arc length function. Why is s

P¸

y=≈- ln x 1

negative when is less than ? x 1 8

s(x)=≈+ ln x-1 8

FIGURE 8

FIGURE 9

8.1 EXERCISES 1. Use the arc length formula (3) to find the length of the curve

15. 2 , 1 0艋x艋 2

y 苷 2x ⫺ 5 , ⫺ 1艋x艋3 . Check your answer by noting that the curve is a line segment and calculating its length by the

16. y 苷 sx ⫺ x 2 ⫹ ⫺ sin 1 ( sx )

distance formula.

17. y苷e x , 0艋x艋1

2. Use the arc length formula to find the length of the curve y 苷 s2 ⫺ x 2 , 0艋x艋1 . Check your answer by noting that

the curve is part of a circle.

18. y 苷 ln x

, a艋x艋b , a⬎ 0

3–6 Set up, but do not evaluate, an integral for the length of the curve.

3. y 苷 cos x , 0 艋 x 艋 2␲ ; 19 – 20 Find the length of the arc of the curve from point to P

point . Q

1 1 y 苷 xe 1 , 0艋x艋1 19.

4. ⫺x 2

2 x y苷 2 , P ( ⫺ 1, 2 ) , Q ( 1, 2 )

x 2 x苷y⫹y 3 , 1艋y艋4 20. , P

6. x a 2 ⫹

y 2 b 2 苷1

; 21– 22 Graph the curve and visually estimate its length. Then find

its exact length.

7–18 Find the length of the curve.

21. 2 2 y苷 3 3 ⫺ 1 , 1艋x艋3

7. y 苷 1 ⫹ 6x 3 , 0艋x艋1

, 0艋x艋2 , y⬎ 0 6 2x

9. x ⫹ y苷 1 3 , 1艋x艋2 6 10x

23 – 26 Use Simpson’s Rule with n 苷 10 to estimate the arc y 4

length of the curve. Compare your answer with the value of the 10. 1 x苷 integral produced by your calculator. ⫹

, 1艋y艋2

8 4y 2

23. y 苷 xe ⫺x , 0艋x艋5

11. x苷 1 3 sy , 1艋y艋9

24. , x 苷 y ⫹ sy 1艋y艋2

12. , 0艋x艋␲

25. y 苷 sec x , 0艋x艋␲

13. , 0艋x艋␲

y 苷 x ln x , 1艋x艋3

14. y苷3⫹ 2 cosh 2x , 0艋x艋1

SECTION 8.1 ARC LENGTH

3 4⫺x , ; y苷x s 0艋x艋4 . the distance traveled by the prey from the time it is dropped (b) Compute the lengths of inscribed polygons with n苷1 ,, 2 until the time it hits the ground. Express your answer correct

27. (a) Graph the curve

and sides. (Divide the interval into equal subintervals.) 4 to the nearest tenth of a meter.

Illustrate by sketching these polygons (as in Figure 6). (c) Set up an integral for the length of the curve. 38 . The Gateway Arch in St. Louis (see the photo on page 256)

(d) Use your calculator to find the length of the curve to four was constructed using the equation decimal places. Compare with the approximations in

y 苷 211.49 ⫺ 20.96 cosh 0.03291765x part (b).

for the central curve of the arch, where and are measured x y ; 28. Repeat Exercise 27 for the curve

in meters and x 艋 91.20 . Set up an integral for the length of the arch and use your calculator to estimate the length

y 苷 x ⫹ sin x

0 艋 x 艋 2␲

correct to the nearest meter.

CAS 29. Use either a computer algebra system or a table of integrals to find the exact length of the arc of the curve 39. y 苷 ln x that lies

A manufacturer of corrugated metal roofing wants to produce between the points

and

panels that are 28 in. wide and 2 in. thick by processing flat sheets of metal as shown in the figure. The profile of the roof-

CAS 30. Use either a computer algebra system or a table of integrals to ing takes the shape of a sine wave. Verify that the sine curve find the exact length of the arc of the curve

y苷x 4 that lies

has equation

and find the width of a flat w

between the points

metal sheet that is needed to make a 28-inch panel. (Use your evaluating the integral, make a substitution that changes the

and

. If your CAS has trouble

calculator to evaluate the integral correct to four significant integral into one that the CAS can evaluate.

digits.)

31. Sketch the curve with equation x 2 ⫹y 2 苷1 and use sym-

metry to find its length.

32. (a) Sketch the curve y 3 苷x 2 .

(b) Use Formulas 3 and 4 to set up two integrals for the arc w 2 in length from 28 in to . Observe that one of these is

an improper integral and evaluate both of them. (c) Find the length of the arc of this curve from 40. (a) The figure shows a telephone wire hanging between

to . two poles at x 苷 ⫺b and x苷b . It takes the shape of a catenary with equation

. Find the 33. Find the arc length function for the curve

y 苷 2x 3 with

length of the wire.

(b) Suppose two telephone poles are 50 ft apart and the length of the wire between the poles is 51 ft. If the lowest ; 34. (a) Graph the curve

starting point P 0 .

point of the wire must be 20 ft above the ground, how (b) Find the arc length function for this curve with starting

1 y苷 3

3 x ⫹ 1 , x⬎ 0 .

high up on each pole should the wire be attached?

point . P 0 ( 1, 7 12 )

(c) Graph the arc length function.

35. Find the arc length function for the curve

⫺ y 苷 sin 1 x⫹ s1 ⫺ x 2 with starting point

36. A steady wind blows a kite due west. The kite’s height above ground from horizontal position x苷0 to x 苷 80 ft is given

_b

by y 苷 150 ⫺ 40 . Find the distance traveled by the

kite.

41. Find the length of the curve

37. A hawk flying at

15 m at an altitude of 180 m accidentally

drops its prey. The parabolic trajectory of the falling prey is

1艋x艋4 described by the equation

y苷 x x st 3 ⫺ 1 dt

x 2 ; 42. The curves with equations x n ⫹y n 苷1 , n苷4 , , , . . . , are 6 8

y 苷 180 ⫺

called fat circles. Graph the curves with n苷2 , , , , and 4 6 8 45 10 to see why. Set up an integral for the length L 2k of the fat

until it hits the ground, where is its height above the ground y circle with n 苷 2k . Without attempting to evaluate this inte- and is the horizontal distance traveled in meters. Calculate x

gral, state the value of lim k l ⬁ L 2k .

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION

DISCOVERY

ARC LENGTH CONTEST

PROJECT The curves shown are all examples of graphs of continuous functions that have the following f

properties. 1. f

2. f 3. The area under the graph of from 0 to 1 is equal to 1. f

The lengths of these curves, however, are different. L

0 1 x LÅ3.249

LÅ3.213 Try to discover formulas for two functions that satisfy the given conditions 1, 2, and 3. (Your

LÅ2.919

LÅ3.152

graphs might be similar to the ones shown or could look quite different.) Then calculate the arc length of each graph. The winning entry will be the one with the smallest arc length.

8.2 AREA OF A SURFACE OF REVOLUTION

A surface of revolution is formed when a curve is rotated about a line. Such a surface is the lateral boundary of a solid of revolution of the type discussed in Sections 6.2 and 6.3. We want to define the area of a surface of revolution in such a way that it corresponds to our intuition. If the surface area is , we can imagine that painting the surface would A require the same amount of paint as does a flat region with area . A

Let’s start with some simple surfaces. The lateral surface area of a circular cylinder with radius and height is taken to be r h A 苷 2␲rh because we can imagine cutting the cylin-

cut

h der and unrolling it (as in Figure 1) to obtain a rectangle with dimensions 2␲r and . h

Likewise, we can take a circular cone with base radius and slant height , cut it along r l the dashed line in Figure 2, and flatten it to form a sector of a circle with radius and cen- l tral angle

. We know that, in general, the area of a sector of a circle with radius

l 1 and angle is ␪ 2

2 l ␪ (see Exercise 35 in Section 7.3) and so in this case the area is

Therefore we define the lateral surface area of a cone to be

A 苷 ␲rl .

SECTION 8.2 AREA OF A SURFACE OF REVOLUTION

What about more complicated surfaces of revolution? If we follow the strategy we used with arc length, we can approximate the original curve by a polygon. When this polygon is rotated about an axis, it creates a simpler surface whose surface area approximates the actual surface area. By taking a limit, we can determine the exact surface area.

The approximating surface, then, consists of a number of bands, each formed by rotat- ing a line segment about an axis. To find the surface area, each of these bands can be considered a portion of a circular cone, as shown in Figure 3. The area of the band (or frus-

l¡

tum of a cone) with slant height and upper and lower radii l r 1 and r 2 is found by sub-

tracting the areas of two cones:

From similar triangles we have

l 1 l 1 ⫹l

r™

which gives

Putting this in Equation 1, we get

2 A 苷 2␲rl

2 1 ⫹r 2 (a) Surface of revolution is the average radius of the band. Now we apply this formula to our strategy. Consider the surface shown in Figure 4,

where

r苷 1

which is obtained by rotating the curve

, a艋x艋b , about the -axis, where is x f

y PP

i i-1

positive and has a continuous derivative. In order to define its surface area, we divide the

P¸

interval into n subintervals with endpoints x 0 ,x 1 ,...,x n and equal width ⌬x , as we

did in determining arc length. If y i

i , then the point P i i ,y i lies on the curve. The

part of the surface between x i⫺ 1 and x i is approximated by taking the line segment P i⫺ 1 P i and rotating it about the -axis. The result is a band with slant height x l苷 P i⫺ 1 P i and aver-

age radius

r苷 1

2 i⫺ 1 ⫹y i so, by Formula 2, its surface area is

(b) Approximating band

y i⫺ 1 ⫹y i

P i⫺ P i

FIGURE 4

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION

As in the proof of Theorem 8.1.2, we have

⫺ 1 2 ⱍ P i P i ⱍ 苷 s1 ⫹ 关 f ⬘共x *兲兴 i ⌬x

where x * i is some number in 关x i ⫺ 1 ,x i 兴 . When ⌬x is small, we have y i 苷 f 共x i 兲 ⬇ f 共x i *兲 and also , since y i ⫺ 1 苷 f 共x i ⫺ 1 兲 ⬇ f 共x *兲 i f is continuous. Therefore

y i ⫺ 1 ⫹y i

ⱍ P i ⫺ 1 P i ⱍ ⬇ 2␲ f 共x i *兲 s1 ⫹ 关 f ⬘共x i *兲兴 2 2 ⌬x

and so an approximation to what we think of as the area of the complete surface of revo- lution is

3 兺 2␲ f 共x i *兲 s1 ⫹ 关 f ⬘共x i *兲兴 2 ⌬x

i苷1

This approximation appears to become better as nl⬁ and, recognizing (3) as a Riemann sum for the function

t共x兲 苷 2␲ f 共x兲 s1 ⫹ 关 f ⬘共x兲兴 2 , we have

lim 2␲ f 共x *兲 i s1 ⫹ 关 f ⬘共x *兲兴 i 2 n b ⌬x 苷 y a 2␲ f 共x兲 s1 ⫹ 关 f ⬘共x兲兴 兺 2 dx

l ⬁ i苷1

Therefore, in the case where is positive and has a continuous derivative, we define the f surface area of the surface obtained by rotating the curve y 苷 f 共x兲 , a 艋x艋b , about

the -axis x as

4 b S苷 y

2␲ f 共x兲 s1 ⫹ 关 f ⬘共x兲兴 a 2 dx

With the Leibniz notation for derivatives, this formula becomes

冑 冉 dx 冊

y a 2␲ y 1⫹

If the curve is described as x 苷 t共y兲 , c 艋y艋d , then the formula for surface area

becomes

冑 冉 dy 冊

6 S苷 dx y

c 2␲ y 1⫹

dy

and both Formulas 5 and 6 can be summarized symbolically, using the notation for arc length given in Section 8.1, as

y 2␲ y ds

7 S苷

SECTION 8.2 AREA OF A SURFACE OF REVOLUTION

For rotation about the -axis, the surface area formula becomes y

y 2␲ x ds

8 S苷

where, as before, we can use either

冑 冉 dx 冊

冑 冉 dy 冊

These formulas can be remembered by thinking of 2␲ y or 2␲ x as the circumference of a circle traced out by the point 共x, y兲 on the curve as it is rotated about the -axis or -axis, x y respectively (see Figure 5).

(x, y) y

x (x, y)

circumference =2πy

circumference =2πx 0 x

FIGURE 5

(a) Rotation about x-axis: S= j 2πy ds

(b) Rotation about y-axis: S=j 2πx ds

V EXAMPLE 1 The curve y 苷 s4 ⫺ x 2 , ⫺ 1艋x艋1 , is an arc of the circle x 2 ⫹y 2 苷4 . Find the area of the surface obtained by rotating this arc about the -axis. (The surface is x

a portion of a sphere of radius 2. See Figure 6.)

y SOLUTION We have

dy

2 共4 ⫺ x 苷 2 兲 ⫺ 1 兾2 dx 共⫺2x兲 苷

1 ⫺x

s4 ⫺ x 2

and so, by Formula 5, the surface area is

⫺ 1 冑 冉 dx 冊

S苷 y 2␲ y 1⫹

1 dy 2

dx

⫺ 1 冑 4⫺x 2

苷 2␲ y s4 ⫺ x 2 1⫹

dx

y ⫺ 1 s4 ⫺ x 2

N Figure 6 shows the portion of the sphere

苷 4␲ y 1 1 dx 苷 4␲共2兲 苷 8␲

whose surface area is computed in Example 1.

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION

N Figure 7 shows the surface of revolution

V EXAMPLE 2 The arc of the parabola y苷x 2 from 共1, 1兲 to 共2, 4兲 is rotated about the

whose area is computed in Example 2.

y -axis. Find the area of the resulting surface.

SOLUTION 1 Using

we have, from Formula 8,

S苷 y 2␲ x ds

1 冑 冉 dx 冊

苷 y 2␲ x 1⫹

Substituting u 苷 1 ⫹ 4x 2 , we have du 苷 8x dx . Remembering to change the limits of

integration, we have

␲ 17 ␲ 2 3 兾2 17 S苷 y su 4 5 du 苷 [ 4 3 u ] 5

N As a check on our answer to Example 2,

17 s17 ⫺ 苷 5 s5 )

notice from Figure 7 that the surface area

should be close to that of a circular cylinder with the same height and radius halfway between

SOLUTION 2 Using

the upper and lower radius of the surface: 2␲ 共1.5兲共3兲 ⬇ 28.27 . We computed that

dx

the surface area was

6 ( 17 s17 ⫺ 5 s5 ) ⬇ 30.85

we have

which seems reasonable. Alternatively, the sur- face area should be slightly larger than the area

冑 冉 dy 冊

S苷 y 2␲ x ds 苷 y 2␲ x 1⫹

4 dx 2

of a frustum of a cone with the same top and

dy

bottom edges. From Equation 2, this is 1

2␲ 共1.5兲 ( s10 ) ⬇ 29.80 .

1 冑 4y dy 苷 ␲ y s4y ⫹ 1

苷 2␲ y

4 y 5 su

du

(where ) u 苷 1 ⫹ 4y

苷 ( 17 s17 ⫺ 5 s5 ) (as in Solution 1)

V EXAMPLE 3 Find the area of the surface generated by rotating the curve y苷e x ,

0艋x艋1 , about the -axis. x

N Another method: Use Formula 6 with

SOLUTION Using Formula 5 with

x 苷 ln y .

dy

y苷e x and

dx 苷e

SECTION 8.2 AREA OF A SURFACE OF REVOLUTION

we have

冑 冉 dx 冊 y 0

y s1 ⫹ u du (where )

1 u苷e

3 ⫺ 苷 2␲ 1 y

sec ␪ d␪

(where and ) u 苷 tan ␪ ␣ 苷 tan e

N Or use Formula 21 in the Table of Integrals.

2 [ sec ␪ tan ␪ ⫹ ln ⱍ sec ␪ ⫹ tan ␪ ⱍ] ␲ 兾4 (by Example 8 in Section 7.2) sec ␣ tan ␣ ⫹ ln 共sec ␣ ⫹ tan ␣兲 ⫺ s2 苷␲ [ ⫺ ln ( s2 ⫹ 1 ) ]

Since , we 2 2 tan ␣ 苷 e 2 have sec ␣ 苷 1 ⫹ tan ␣苷1⫹e and

S苷␲ [ e s1 ⫹ e 2 ⫹ ln ( e ⫹ s1 ⫹ e 2 ) ⫺ s2 ⫺ ln ( s2 ⫹ 1 ) ]

8.2 EXERCISES 1– 4 Set up, but do not evaluate, an integral for the area of the

17– 20 Use Simpson’s Rule with n 苷 10 to approximate the area surface obtained by rotating the curve about (a) the -axis and x

of the surface obtained by rotating the curve about the -axis. x (b) the -axis. y

Compare your answer with the value of the integral produced by 1. 4

y苷x , 0艋x艋1

2. y 苷 xe ⫺x , 1艋x艋3

your calculator.

3. y 苷 tan ⫺ 1 17. x 18. , 1艋x艋3 , 1艋x艋2

, 0艋x艋1

4. x 苷 sy ⫺ y 2 y 苷 ln x

y 苷 x ⫹ sx 19. y 苷 sec x 2 , 0艋x艋␲ 兾3 20. y苷e ⫺x , 0艋x艋1

5 –12 Find the area of the surface obtained by rotating the curve about the -axis. CAS x 21– 22 Use either a CAS or a table of integrals to find the exact

area of the surface obtained by rotating the given curve about the 5. y苷x 3 , 0艋x艋2

x -axis.

6. 9x 苷 y 2 ⫹ 18 , 2艋x艋6

22. y 苷 sx 2 ⫹ 1 , 0艋x艋3 7. y 苷 s1 ⫹ 4x , 1艋x艋5

21. y 苷 1兾x , 1艋x艋2

8. y 苷 c ⫹ a cosh共x兾a兲 , 0艋x艋a

CAS

23 – 24 Use a CAS to find the exact area of the surface obtained

by rotating the curve about the -axis. If your CAS has trouble y y 苷 sin ␲ x , 0艋x艋1

evaluating the integral, express the surface area as an integral in

10. x

1 1 the other variable.

y苷 ⫹ , 2 艋x艋 1 6 2x

24. y 苷 ln共x ⫹ 1兲 , 0艋x艋1 11. x苷 1 3 共y 2 ⫹ 2 兲 3 兾2 , 1艋y艋2

23. y苷x 3 , 0艋y艋1

12. x 苷 1 ⫹ 2y 2 , 1艋y艋2

25. If the region ᏾ 苷 兵共x, y兲 ⱍ x 艌 1, 0 艋 y 艋 1 兾x其 is rotated about the -axis, the volume of the resulting solid is finite x

(see Exercise 63 in Section 7.8). Show that the surface area is 13 –16 The given curve is rotated about the -axis. Find the area y

infinite. (The surface is shown in the figure and is known as of the resulting surface.

Gabriel’s horn .)

14. 2 y= y苷1⫺x 1 , 0艋x艋1 x

13. y苷s 3 x , 1艋y艋2

16. y苷 , 1艋x艋2 1 x

15. x 苷 sa 2 ⫺y 2 , 0艋y艋a 兾2

4 x 2 ⫺ 1 2 ln x

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION

CAS 32. Use the result of Exercise 31 to set up an integral to find the x -axis, find the area of the resulting surface.

26. If the infinite curve y苷e ⫺x , x 艌 0 , is rotated about the

area of the surface generated by rotating the curve , y 苷 sx 27. (a) If a ⬎ 0 , find the area of the surface generated by rotating

0艋x艋4 , about the line y苷4 . Then use a CAS to evaluate

the loop of the curve 3ay 2 苷 x共a ⫺ x兲 2 about the -axis. x

the integral.

(b) Find the surface area if the loop is rotated about the y 33. -axis. Find the area of the surface obtained by rotating the circle

x 2 ⫹y 2 苷r 2 about the line y苷r . 28. A group of engineers is building a parabolic satellite dish

34. Show that the surface area of a zone of a sphere that lies about the -axis. If the dish is to have a 10-ft diameter and a y

whose shape will be formed by rotating the curve

y 苷 ax 2

between two parallel planes is S 苷 ␲ dh , where is the diam- d maximum depth of 2 ft, find the value of and the surface a eter of the sphere and is the distance between the planes. h

area of the dish. (Notice that depends only on the distance between the S 29. (a) The ellipse

planes and not on their location, provided that both planes intersect the sphere.)

a ⬎b

a b 35. Formula 4 is valid only when f 共x兲 艌 0 . Show that when is rotated about the -axis to form a surface called an x

f 共x兲 is not necessarily positive, the formula for surface area ellipsoid, or prolate spheroid . Find the surface area of this

becomes

ellipsoid. (b) If the ellipse in part (a) is rotated about its minor axis (the

S苷 b

y a 2␲ ⱍ f 共x兲 ⱍ s1 ⫹ 关 f ⬘共x兲兴 dx

y -axis), the resulting ellipsoid is called an oblate spheroid.

Find the surface area of this ellipsoid.

Let be the length of the curve L y 苷 f 共x兲 , a 艋x艋b , where Find the surface area of the torus in Exercise 63 in f is positive and has a continuous derivative. Let S f be the Section 6.2. surface area generated by rotating the curve about the -axis. x

31. If the curve y 苷 f 共x兲 , a 艋x艋b , is rotated about the

If is a positive constant, define c t共x兲 苷 f 共x兲 ⫹ c and let S t

be the corresponding surface area generated by the curve area of the resulting surface.

horizontal line y苷c , where f 共x兲 艋 c , find a formula for the

y 苷 t共x兲 , a 艋x艋b . Express S t in terms of S f and . L

DISCOVERY

ROTATING ON A SLANT

PROJECT We know how to find the volume of a solid of revolution obtained by rotating a region about a

horizontal or vertical line (see Section 6.2). We also know how to find the surface area of a sur- face of revolution if we rotate a curve about a horizontal or vertical line (see Section 8.2). But what if we rotate about a slanted line, that is, a line that is neither horizontal nor vertical? In this project you are asked to discover formulas for the volume of a solid of revolution and for the area of a surface of revolution when the axis of rotation is a slanted line.

Let C be the arc of the curve y 苷 f 共x兲 between the points P 共 p, f 共 p兲兲 and Q 共q, f 共q兲兲 and let ᏾ be the region bounded by , by the line C y 苷 mx ⫹ b (which lies entirely below ), and by the C

perpendiculars to the line from and . P Q

y=ƒ ᏾

P C y=mx+b

Îu

SECTION 8.3 APPLICATIONS TO PHYSICS AND ENGINEERING

1. Show that the area of ᏾ is

2 y p 关 f 共x兲 ⫺ mx ⫺ b兴关1 ⫹ mf ⬘共x兲兴 dx 1⫹m

[Hint: This formula can be verified by subtracting areas, but it will be helpful throughout the project to derive it by first approximating the area using rectangles perpendicular to the line, as shown in the figure. Use the figure to help express ⌬u in terms of ⌬x .]

tangent to C

at {x i , f(x i )}

y=mx+b Îu

Îx

y (2π, 2π) 2. Find the area of the region shown in the figure at the left. 3. Find a formula similar to the one in Problem 1 for the volume of the solid obtained by

rotating ᏾ about the line y 苷 mx ⫹ b y=x+sin x . 4. y=x-2 Find the volume of the solid obtained by rotating the region of Problem 2 about the

line . y苷x⫺2

5. Find a formula for the area of the surface obtained by rotating C about the line y 苷 mx ⫹ b .

CAS 6. Use a computer algebra system to find the exact area of the surface obtained by rotating the curve

y 苷 sx 1 , 0艋x艋4 , about the line y苷 2 x . Then approximate your result to three

decimal places.

8.3 APPLIC ATIONS TO PHYSICS AND ENGINEERING Among the many applications of integral calculus to physics and engineering, we consider

two here: force due to water pressure and centers of mass. As with our previous applica- tions to geometry (areas, volumes, and lengths) and to work, our strategy is to break up the physical quantity into a large number of small parts, approximate each small part, add the results, take the limit, and then evaluate the resulting integral.

HYDROSTATIC FORCE AND PRESSURE

surface of fluid

Deep-sea divers realize that water pressure increases as they dive deeper. This is because the weight of the water above them increases.

In general, suppose that a thin horizontal plate with area square meters is submerged A in a fluid of density kilograms per cubic meter at a depth meters below the surface of ␳ d the fluid as in Figure 1. The fluid directly above the plate has volume

V 苷 Ad , so its mass is m 苷 ␳V 苷 ␳Ad . The force exerted by the fluid on the plate is therefore

FIGURE 1

F 苷 mt 苷 ␳tAd

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION

where is the acceleration due to gravity. The pressure t P on the plate is defined to be the force per unit area:

P苷

A 苷 ␳td

N When using US Customary units, we write

The SI unit for measuring pressure is newtons per square meter, which is called a pascal

P 苷 ␳td 苷 ␦d , where ␦ 苷 ␳t is the weight

(abbreviation: 1 N 兾m 2 苷1 Pa). Since this is a small unit, the kilopascal (kPa) is often

density (as opposed to , which is the mass ␳

used. For instance, because the density of water is

␳ 苷 1000 kg兾m , the pressure at the

density). For instance, the weight density of

water is ␦ 苷 62.5 lb兾ft 3 .

bottom of a swimming pool 2 m deep is

9.8 m 兾s P 苷 ␳td 苷 1000 kg兾m 2 ⫻ 2m

苷 19,600 Pa 苷 19.6 kPa

An important principle of fluid pressure is the experimentally verified fact that at any point in a liquid the pressure is the same in all directions. (A diver feels the same pressure on nose and both ears.) Thus the pressure in any direction at a depth in a fluid with mass d density is ␳ given by

1 P 苷 ␳td 苷 ␦d

This helps us determine the hydrostatic force against a vertical plate or wall or dam in a fluid. This is not a straightforward problem because the pressure is not constant but increases as the depth increases.

50 m

V EXAMPLE 1

A dam has the shape of the trapezoid shown in Figure 2. The height is

20 m, and the width is 50 m at the top and 30 m at the bottom. Find the force on the dam due to hydrostatic pressure if the water level is 4 m from the top of the dam.

20 m

SOLUTION We choose a vertical -axis with origin at the surface of the water as in x Figure 3(a). The depth of the water is 16 m, so we divide the interval 关0, 16兴 into sub-

30 m

intervals of equal length with endpoints x i and we choose x i * 僆 关x i ⫺ 1 ,x i 兴 . The i th hori-

FIGURE 2

zontal strip of the dam is approximated by a rectangle with height ⌬x and width , w i where, from similar triangles in Figure 3(b),

w i 苷 2共15 ⫹ a兲 苷 2

15 ⫹ 8 ⫺ ( 1 2 x * i ) 苷 46 ⫺ x i *

15 If A i is the area of the i th strip, then

x (a)

A i ⬇ w i ⌬x 苷 共46 ⫺ x i *兲 ⌬x

10 If ⌬x is small, then the pressure P i on the i th strip is almost constant and we can use Equation 1 to write

a 20

P i ⬇ 1000 tx * i

16-x i *

The hydrostatic force F i acting on the i th strip is the product of the pressure and the

F i 苷P i A i ⬇ 1000 tx i *共46 ⫺ x i *兲 ⌬x

SECTION 8.3 APPLICATIONS TO PHYSICS AND ENGINEERING

Adding these forces and taking the limit as nl⬁ , we obtain the total hydrostatic force on the dam:

n n F 苷 lim l 1000 ⬁ 兺 tx *共46 ⫺ x i *兲 ⌬x i苷1 i

苷 16 y

0 tx共46 ⫺ x兲 dx

苷 1000共9.8兲 16 y 共46x ⫺ x

EXAMPLE 2 Find the hydrostatic force on one end of a cylindrical drum with radius 3 ft if the drum is submerged in water 10 ft deep.

SOLUTION In this example it is convenient to choose the axes as in Figure 4 so that the origin is placed at the center of the drum. Then the circle has a simple equation,

x 2 ⫹y 2 苷9 . As in Example 1 we divide the circular region into horizontal strips of

d dd d ii i i

7 7777 œœ„ œœ„ œœœœ œœ œ œ œœ œœœœœœ œ„ œ„„„„„„„ œ„ œ œœ ( y i )

equal width. From the equation of the circle, we see that the length of the i th strip is

2 s9 ⫺ 共y i *兲 2 and so its area is

The pressure on this strip is approximately

≈ ≈ ≈ ≈ ≈ ≈ + + + + ¥ ¥ ¥ ¥ ¥ ¥ =9 =9 =99 9 ␦d i 苷 62.5共7 ⫺ y *兲 i

FIGURE 4

and so the force on the strip is approximately

␦d i A i

*兲2 i s9 ⫺ 共y i *兲 苷 62.5共7 ⫺ y 2 ⌬y

The total force is obtained by adding the forces on all the strips and taking the limit:

n F 苷 lim 62.5 共7 ⫺ y i *兲2 s9 ⫺ 共y i *兲 2 l ⌬y ⬁ i苷1 兺

苷 125 3 共7 ⫺ y兲 s9 ⫺ y y 2

dy

dy ⫺ 3 ⫺ 3 s9 ⫺ y The second integral is 0 because the integrand is an odd function (see Theorem 5.5.7).

2 y 3 s9 ⫺ y dy ⫺ 125 y

The first integral can be evaluated using the trigonometric substitution y 苷 3 sin ␪ , but it’s simpler to observe that it is the area of a semicircular disk with radius 3. Thus

2 ␲ 共3兲 y 2 ⫺ 3

F 苷 875 3 s9 ⫺ y 2 dy 苷 875 ⴢ 1

⬇ 12,370 lb

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION

MOMENTS AND CENTERS OF MASS Our main objective here is to find the point on which a thin plate of any given shape bal- P

ances horizontally as in Figure 5. This point is called the center of mass (or center of grav- ity) of the plate.

We first consider the simpler situation illustrated in Figure 6, where two masses m 1 and m 2 are attached to a rod of negligible mass on opposite sides of a fulcrum and at distances

d 1 and d 2 from the fulcrum. The rod will balance if

This is an experimental fact discovered by Archimedes and called the Law of the Lever. (Think of a lighter person balancing a heavier one on a seesaw by sitting farther away from

m¡

m™

the center.)

Now suppose that the rod lies along the -axis with x m 1 at x 1 and m 2 at x 2 and the center of mass at . If we compare Figures 6 and 7, we see that x d 1 苷x⫺x 1 and d 2 苷x 2 ⫺x and

fulcrum

so Equation 2 gives

The numbers and are m 1 x 1 m 2 x 2 called the moments of the masses and (with m 1 m 2 respect to the origin), and Equation 3 says that the center of mass is obtained by adding the x moments of the masses and dividing by the total mass m苷m 1 ⫹m 2 .

In general, if we have a system of particles with masses n m 1 , m 2 , ..., m n located at the points , x 1 x 2 , ..., x n on the -axis, it x can be shown similarly that the center of mass of the system is located at

i苷1 兺 m

x苷 n

4 i苷1

i苷1 兺

where m苷 冘 m i is the total mass of the system, and the sum of the individual moments

M苷

i苷1 兺

is called the moment of the system about the origin. Then Equation 4 could be rewritten as mx 苷 M , which says that if the total mass were considered as being concentrated at the center of mass , then its moment would be the same as the moment of the system. x

SECTION 8.3 APPLICATIONS TO PHYSICS AND ENGINEERING

Now we consider a system of particles with masses n m 1 , m 2 , ..., m n located at the

m¡

points , 共x 1 ,y 1 兲 共x 2 ,y 2 兲 ,..., 共x n ,y n 兲 in the xy -plane as shown in Figure 8. By analogy with

m£ y£

the one-dimensional case, we define the moment of the system about the y-axis to be

m™ 5

i苷1 兺

and the moment of the system about the x-axis as

FIGURE 8

i苷1 兺 m

Then M y measures the tendency of the system to rotate about the -axis and y M x measures the tendency to rotate about the -axis. x

As in the one-dimensional case, the coordinates 共x, y兲 of the center of mass are given in terms of the moments by the formulas

x苷

y苷

where m苷 冘 m i is the total mass. Since mx 苷 M y and my 苷 M x , the center of mass 共x, y兲 is the point where a single particle of mass m would have the same moments as the system.

V EXAMPLE 3 Find the moments and center of mass of the system of objects that have masses 3, 4, and 8 at the points 共⫺1, 1兲 , 共2, ⫺1兲 , and 共3, 2兲 , respectively.

SOLUTION We use Equations 5 and 6 to compute the moments: M y 苷 3共⫺1兲 ⫹ 4共2兲 ⫹ 8共3兲 苷 29

y center of mass

8 M x 苷 3共1兲 ⫹ 4共⫺1兲 ⫹ 8共2兲 苷 15

3 Since m 苷 3 ⫹ 4 ⫹ 8 苷 15 , we use Equations 7 to obtain

Thus the center of mass is 1 ( 14 15 ,1 ) . (See Figure 9.)

Next we consider a flat plate (called a lamina) with uniform density that occupies a ␳ region ᏾ of the plane. We wish to locate the center of mass of the plate, which is called the centroid of . In doing so we use the following physical principles: The symmetry ᏾ principle says that if ᏾ is symmetric about a line , then the centroid of l ᏾ lies on . (If l ᏾ is reflected about , then l ᏾ remains the same so its centroid remains fixed. But the only fixed points lie on .) Thus the centroid of a rectangle is its center. Moments should be l defined so that if the entire mass of a region is concentrated at the center of mass, then its moments remain unchanged. Also, the moment of the union of two nonoverlapping regions should be the sum of the moments of the individual regions.

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION y

y=ƒ Suppose that the region ᏾ is of the type shown in Figure 10(a); that is, ᏾ lies between the lines x苷a and x苷b , above the -axis, and beneath the graph of , where x f f is a continuous function. We divide the interval 关a, b兴 into n subintervals with endpoints x 0 ,x 1 , ..., x n and equal width ⌬x . We choose the sample point x * i to be the midpoint of x i

᏾ the th subinterval, that is, i x i 苷 共x i ⫺ 1 ⫹x i 兲兾2 . This determines the polygonal approxima- tion to ᏾ shown in Figure 10(b). The centroid of the th approximating rectangle i R i is its

0 兲 ⌬x

center C i ( x i , 1 2 f 共x i 兲 ) . Its area is f 共x i , so its mass is

The moment of R i about the -axis is the product of its mass and the distance from y C i to

1 the -axis, which is y x i C . Thus

i ”x i , f(x 2 i )’

M y 共R i 兲 苷 关␳f 共x i 兲 ⌬x兴 x i 苷 ␳x i f 共x i 兲 ⌬x

Adding these moments, we obtain the moment of the polygonal approximation to , and ᏾ then by taking the limit as nl⬁ we obtain the moment of ᏾ itself about the -axis: y

R¡ R™ x i_1 x i R£ n x i

y 苷 lim ␳x i n f ⬁ 兺 共x i 兲 ⌬x 苷 ␳ y xf

In a similar fashion we compute the moment of R i about the -axis as the product of its x

mass and the distance from C i to the -axis: x M 1

2 兲苷␳ⴢ 2 关 f 共x i 兲兴 ⌬x Again we add these moments and take the limit to obtain the moment of ᏾ about the

x -axis:

1 苷 lim 1 n

l ⬁ 兺 ␳ⴢ 2 关 f 共x

i 兲兴 2 ⌬x 苷 ␳ 2 2 关 f 共x兲兴 dx

i苷1

Just as for systems of particles, the center of mass of the plate is defined so that mx 苷 M y and my 苷 M x . But the mass of the plate is the product of its density and its area:

m 苷 ␳A 苷 ␳ b y

a f 共x兲 dx and so

y a xf 共x兲 dx y a xf 共x兲 dx

x苷 m 苷

␳ y a f 共x兲 dx y a f 共x兲 dx

y a 2 关 f 共x兲兴 2 M dx x

b 1 2 b dx ␳ 1 y

a 2 关 f 共x兲兴

y苷 m 苷

y a f 共x兲 dx

a f 共x兲 dx

Notice the cancellation of the ’s. The location of the center of mass is independent of the ␳ density.

SECTION 8.3 APPLICATIONS TO PHYSICS AND ENGINEERING

In summary, the center of mass of the plate (or the centroid of ) is located at the point ᏾ 共x, y兲 , where

8 1 b 1 x苷 b y 1

y苷 y a 2 关 f 共x兲兴 2 dx

a xf 共x兲 dx

EXAMPLE 4 Find the center of mass of a semicircular plate of radius . r

SOLUTION In order to use (8) we place the semicircle as in Figure 11 so that

y= r@-≈ œ„„„„„

f 共x兲 苷 sr 2 ⫺x 2 and a 苷 ⫺r , b苷r . Here there is no need to use the formula to calcu- late because, by the symmetry principle, the center of mass must lie on the -axis, so ” x 0, 4r y

x苷0 1 A苷 ␲r 2

. The area of the semicircle is

2 , so

_r 0 r

y苷 y 1 2 ⫺r 关 f 共x兲兴 2 A dx

FIGURE 11

␲r 2 ⴢ 2 y

⫺r ( sr

⫺x

) dx

2 y 共r 2 ⫺x 2 兲 dx 苷 2 r 2 x ␲r ⫺ 0 ␲r 冋 3 册

2 2r 3 4r

2 3 ␲r 苷 3␲

The center of mass is located at the point 共0, 4r兾共3␲兲兲 .

EXAMPLE 5 Find the centroid of the region bounded by the curves y 苷 cos x , y苷0 , x苷0 , and . x 苷 ␲兾2 SOLUTION The area of the region is

␲ A苷 兾2 y

0 cos x dx 苷 sin x ] 0 苷1

so Formulas 8 give

␲ x苷 兾2 xf x cos x dx

A y 0 共x兲 dx 苷 y 0

␲ 苷 x sin x 兾2 ]

0 ⫺ y sin x dx (by integration by parts) 0

1 2 1 ␲ y=cos x 兾2

A 0 2 关 f 共x兲兴 dx 苷 2 y 0

y苷 2 y cos x dx

” -1, ’ π π 2 8 1 ␲ 兾2

4 y 共1 ⫹ cos 2x兲 dx 苷 4 [ x ⫹ 0 1 2 sin 2x ] 0

FIGURE 12

The centroid is 1 ( 1 2 ␲ ⫺ 1, 8 ␲ ) and is shown in Figure 12.

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION y

C ”x , 1 关 f(x )+g(x )兴’

If the region ᏾ lies between two curves y 苷 f 共x兲 and

y 苷 t共x兲

, where

f 共x兲 艌 t共x兲 , as

iii

2 2 illustrated in Figure 13, then the same sort of argument that led to Formulas 8 can be used

y=ƒ

to show that the centroid of ᏾ is 共x, y兲 , where

1 y=© b

9 x苷 y

a x 关 f 共x兲 ⫺ A t共x兲兴 dx

y苷

1 2 兵关 f 共x兲兴 2 ⫺ 关

t共x兲兴 2 其 dx

FIGURE 13

(See Exercise 47.) EXAMPLE 6 Find the centroid of the region bounded by the line y苷x and the

parabola . 2 y苷x

SOLUTION The region is sketched in Figure 14. We take f 共x兲 苷 x 2 , t共x兲 苷 x , a苷0 , and

b苷1 in Formulas 9. First we note that the area of the region is

y=x ” , ’ 1 2

A苷 y 0 共x ⫺ x 2 兲 dx 苷 ⫺

x苷 y x

关 f 共x兲 ⫺ t共x兲兴 dx 苷 1 y 0 共x ⫺ x 兲 dx

苷6 y ⫺x 0 兲 dx 苷 6 ⫺

共x 2 3 x

1 2 2 1 1 y苷 1 ⫺

关 t共x兲兴 其 dx 苷 1 y 0 2 共x

0 2 兵关 f 共x兲兴

The centroid is 1 ( 2 2 , 5 ) .

We end this section by showing a surprising connection between centroids and volumes of revolution.

N This theorem is named after the Greek

THEOREM OF PAPPUS Let ᏾

be a plane region that lies entirely on one side of a

mathematician Pappus of Alexandria, who

line in the plane. If l ᏾ is rotated about , then the volume of the resulting solid is l

lived in the fourth century AD.

the product of the area of A ᏾ and the distance traveled by the centroid of . d ᏾

PROOF We give the proof for the special case in which the region lies between and as in Figure 13 and the line is the -axis. Using the method of cylindrical shells l y

SECTION 8.3 APPLICATIONS TO PHYSICS AND ENGINEERING

(see Section 6.3), we have

V苷 b y

a 2␲ x 关 f 共x兲 ⫺ t共x兲兴 dx

苷 2␲ b y

a x 关 f 共x兲 ⫺ t共x兲兴 dx

(by Formulas 9)

where d 苷 2␲x is the distance traveled by the centroid during one rotation about the y -axis.

V EXAMPLE 7

A torus is formed by rotating a circle of radius about a line in the plane r

of the circle that is a distance R

from the center of the circle. Find the volume of

the torus. SOLUTION The circle has area

A 苷 ␲r 2 . By the symmetry principle, its centroid is its cen- ter and so the distance traveled by the centroid during a rotation is d 苷 2␲R . Therefore, by the Theorem of Pappus, the volume of the torus is

The method of Example 7 should be compared with the method of Exercise 63 in Section 6.2.

8.3 EXERCISES 1. An aquarium 5 ft long, 2 ft wide, and 3 ft deep is full of

5. 6m 6m

water. Find (a) the hydrostatic pressure on the bottom of the

1m

aquarium, (b) the hydrostatic force on the bottom, and (c) the hydrostatic force on one end of the aquarium.

2. A tank is 8 m long, 4 m wide, 2 m high, and contains kerosene

with density 820 kg 兾m 3 to a depth of 1.5 m. Find (a) the hydro-

static pressure on the bottom of the tank, (b) the hydrostatic force on the bottom, and (c) the hydrostatic force on one end

8. 4m of the tank.

7. 2m

1m

A vertical plate is submerged (or partially submerged) in water and has the indicated shape. Explain how to approximate the hydrostatic force against one side of the plate by a Riemann sum. Then express the force as an integral and evaluate it.

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION

11. 2a 19. A vertical, irregularly shaped plate is submerged in water. The table shows measurements of its width, taken at the indi-

cated depths. Use Simpson’s Rule to estimate the force of the water against the plate.

Depth (m)

0 0.8 1.7 2.4 2.9 3.3 3.6 12. A large tank is designed with ends in the shape of the region

Plate width (m)

between the curves 1 y苷

20. (a) Use the formula of Exercise 18 to show that Find the hydrostatic force on one end of the tank if it is filled to a depth of 8 ft with gasoline. (Assume the gasoline’s den- sity is

2 x 2 and y 苷 12 , measured in feet.

F 苷 共␳tx 兲A .)

42.0 lb 兾ft 3

where is the -coordinate of the centroid of the plate x x 13. A trough is filled with a liquid of density 840 kg 兾m . The 3 and is its area. This equation shows that the hydrostatic A

ends of the trough are equilateral triangles with sides 8 m force against a vertical plane region is the same as if the long and vertex at the bottom. Find the hydrostatic force on

region were horizontal at the depth of the centroid of the one end of the trough.

region.

(b) Use the result of part (a) to give another solution to A vertical dam has a semicircular gate as shown in the figure.

Exercise 10.

Find the hydrostatic force against the gate. 21– 22 Point-masses m i are located on the -axis as shown. Find x

2m

the moment M of the system about the origin and the center of

water level

mass . x

12 m

21. m¡=40 m™=30

0 3 7 x 15. A cube with 20-cm-long sides is sitting on the bottom of an

aquarium in which the water is one meter deep. Estimate the hydrostatic force on (a) the top of the cube and (b) one of the

23 – 24 The masses m i are located at the points . Find the P i sides of the cube.

moments M x and M y and the center of mass of the system. 16. A dam is inclined at an angle of 30⬚ from the vertical and has

23. m 1 苷6 , m 2 苷5 , m 3 苷 10 ;

the shape of an isosceles trapezoid 100 ft wide at the top and

P 1 共1, 5兲 , P 2 共3, ⫺2兲 , P 3 共⫺2, ⫺1兲

50 ft wide at the bottom and with a slant height of 70 ft. Find the hydrostatic force on the dam when it is full of water.

24. m 1 苷6 , m 2 苷5 , m 3 苷1 , m 4 苷4 ;

P 1 共1, ⫺2兲 , P 2 共3, 4兲 , P 3 共⫺3, ⫺7兲 , P 4 共6, ⫺1兲 A swimming pool is 20 ft wide and 40 ft long and its bottom

is an inclined plane, the shallow end having a depth of 3 ft and the deep end, 9 ft. If the pool is full of water, estimate

the hydrostatic force on (a) the shallow end, (b) the deep end, 25 – 28 Sketch the region bounded by the curves, and visually esti- (c) one of the sides, and (d) the bottom of the pool.

mate the location of the centroid. Then find the exact coordinates of the centroid.

18. Suppose that a plate is immersed vertically in a fluid with

25. y苷4⫺x 2 , y苷0

density and the width of the plate is ␳ w 共x兲 at a depth of x meters beneath the surface of the fluid. If the top of the plate

26. 3x ⫹ 2y 苷 6 , y苷0 , x苷0

is at depth and the bottom is at depth , show that the a b

hydrostatic force on one side of the plate is

y x苷2

a ␳ tx 共x兲 dx

SECTION 8.3 APPLICATIONS TO PHYSICS AND ENGINEERING

29 –33 Find the centroid of the region bounded by the given 40 – 41 Find the centroid of the region shown, not by integration, curves.

but by locating the centroids of the rectangles and triangles (from 29. y苷x 2 , x苷y 2 Exercise 39) and using additivity of moments.

41. y 31. y 苷 sin x , y 苷 cos x , x苷0 , x 苷 ␲兾4

30. y苷x⫹2 , y苷x 2 40. y

3 2 32. y苷x 3 , x

⫹y苷2 , y苷0 1

33. x苷5⫺y 2 , x苷0 _2

_1 0 1 2 x

34 –35 Calculate the moments M x and M y and the center of mass _1

of a lamina with the given density and shape. 34. ␳苷3

42. A rectangle with sides and is divided into two parts R a b y

R 1 and R 2 by an arc of a parabola that has its vertex at one corner of and passes through the opposite corner. Find the 1 R

centroids of both R 1 and R 2 .

R™

b R¡ 36. Use Simpson’s Rule to estimate the centroid of the region shown.

4 43. If is the -coordinate of the centroid of the region that lies x x under the graph of a continuous function , where f a 艋x艋b ,

2 show that

44 – 46 Use the Theorem of Pappus to find the volume of the ; 37. Find the centroid of the region bounded by the curves y苷2 x

given solid.

and y苷x 2

, 44. 0艋x艋2 , to three decimal places. Sketch A sphere of radius r (Use Example 4.) the region and plot the centroid to see if your answer is

45. A cone with height and base radius h r reasonable. 46. The solid obtained by rotating the triangle with vertices

, and about the x -axis of intersection of the curves y 苷 x ⫹ ln x and y苷x ⫺x .

; 38. Use a graph to find approximate -coordinates of the points x 3 ,

Then find (approximately) the centroid of the region bounded by these curves.

47. Prove Formulas 9.

be the region that lies between the curves y苷x m of intersection of the medians. [Hints: Place the axes so that

39. Prove that the centroid of any triangle is located at the point

48. Let ᏾

and y苷x n , 0艋x艋1 , where m and are integers with n the vertices are

, and

. Recall that a median

0艋n⬍m .

is a line segment from a vertex to the midpoint of the oppo-

(a) Sketch the region . ᏾

site side. Recall also that the medians intersect at a point two- (b) Find the coordinates of the centroid of . ᏾ thirds of the way from each vertex (along the median) to the

(c) Try to find values of m and such that the centroid lies n opposite side.]

outside ᏾ .

CHAPTER 8 FURTHER APPLICATIONS OF INTEGRATION

DISCOVERY

COMPLEMENTARY COFFEE CUPS

PROJECT Suppose you have a choice of two coffee cups of the type shown, one that bends outward and one

inward, and you notice that they have the same height and their shapes fit together snugly. You wonder which cup holds more coffee. Of course you could fill one cup with water and pour it into the other one but, being a calculus student, you decide on a more mathematical approach. Ignoring the handles, you observe that both cups are surfaces of revolution, so you can think of the coffee as a volume of revolution.

x=k

A™ x=f(y)