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SECTION 12.4
APPLICATIONS OF DOUBLE INTEGRALS
P212.4
APPLICATIONS OF DOUBLE INTEGRALS
We have already seen one application of double integrals: computing volumes.
Another geometric application is finding areas of surfaces and this will be done in the next chapter.
In this section we explore physical applications such as Computing mass Electric charge Center of mass Moment of inertia.
P312.4
DENSITY AND MASS
In Chapter 7, we used single integrals to compute moments and the center of mass of a thin plate or lamina with constant density.
Now, equipped with the double integral, we can consider a lamina with variable density.
P412.4
DENSITY
Suppose the lamina occupies a region D of the xy-plane.
Also, let its density (in units of mass per unit area) at a point (x, y) in D be given by (x, y), where is a continuous function on D.
P512.4
MASS
This means that:
where: Δm and ΔA are the mass
and area of a small rectangle that contains (x, y).
The limit is taken as the dimensions of the rectangle approach 0.
See Figure 1.
( , ) limm
x yA
P612.4
MASS
To find the total mass m of the lamina, we: Divide a rectangle R containing D into subrectangles
Rij of equal size. See Figure 2. Consider (x, y) to be 0
outside D.
P712.4
MASS
If we choose a point (xij*, yij*) in Rij , then the mass of the part of the lamina that occupies Rij is approximately
(xij*, yij*) ∆Aij
where ∆Aij is the area of Rij.
P812.4
MASS
If we add all such masses, we get an approximation to the total mass:
* *
1 1
( , )k l
ij ij iji j
m x y A
P912.4
MASS
If we now increase the number of subrectangles, we obtain the total mass m of the lamina as the limiting value of the approximations:
* *
max , 01 1
lim ( , )
( , )
i j
k l
ij ij ijx yi j
D
m x y A
x y dA
P1012.4
DENSITY AND MASS
Physicists also consider other types of density that can be treated in the same manner. For example, an electric charge is distributed over a
region D and the charge density (in units of charge per unit area) is given by (x, y) at a point (x, y) in D.
Then, the total charge Q is given by:( , )
D
Q x y dA
P1112.4
Example 1
Charge is distributed over the triangular region D in Figure 3 so that the charge density at (x, y) is (x, y) = xy, measured in coulombs per square meter (C/m2).
Find the total charge.
P1212.4
Example 1 SOLUTION
From Equation 2 and Figure 3, we have:
1 1
0 1
121
01
( , )
D
x
y
y x
Q x y dA
xy dy dx
yx dx
x
P1312.4
Example 1 SOLUTION
The total charge is C5
24
1 2 2
0
1 2 312 0
13 4
0
[1 (1 ) ]2
(2 )
1 2 5
2 3 4 24
xx dx
x x dx
x x
P1412.4
MOMENTS AND CENTERS OF MASS
In Section 7.6, we found the center of mass of a lamina with constant density.
Here, we consider a lamina with variable density.
Suppose the lamina occupies a region D and has density function (x, y). Recall from Chapter 7 that we defined the moment
of a particle about an axis as the product of its mass and its directed distance from the axis.
P1512.4
MOMENTS AND CENTERS OF MASS
We divide D into small rectangles as in Figure 2.
Then, the mass of Rij is approximately:
(xij*, yij*) ∆Aij
So, we can approximate the moment of Rij with respect to the x-axis by:
[(xij*, yij*) ∆Aij] yij*
P1612.4
MOMENT ABOUT X-AXIS
If we now add these quantities and take the limit as the number of subrectangles becomes large, we obtain the moment of the entire lamina about the x-axis:
* * *
max , 01 1
lim ( , )
( , )
i j
m n
x ij ij ij ijx y
i j
D
M y x y A
y x y dA
P1712.4
MOMENT ABOUT Y-AXIS
Similarly, the moment about the y-axis is:
* * *
max , 01 1
lim ( , )
( , )
i j
m n
y ij ij ij ijx y
i j
D
M x x y A
x x y dA
P1812.4
CENTER OF MASS
As before, we define the center of mass so that and
The physical significance is that: The lamina behaves as if its entire mass is
concentrated at its center of mass.
( , )x y
ymx Mxmy M
P1912.4
CENTER OF MASS
Thus, the lamina balances horizontally when supported at its center of mass.
See Figure 4.
P2012.4
The coordinates of the center of mass of a lamina occupying the region D and having density function (x, y) are:
where the mass m is given by:
Formulas 5
( , )x y
1( , )y
D
Mx x x y dA
m m
( , )D
m x y dA
1( , )x
D
My y x y dA
m m
P2112.4
Example 2
Find the mass and center of mass of a triangular lamina with vertices
(0, 0), (1, 0), (0, 2)
and if the density function is
(x, y) = 1 + 3x + y
P2212.4
Example 2 SOLUTION
The triangle is shown in Figure 5. Note that the equation of the upper boundary is:
y = 2 – 2x
P2312.4
Example 2 SOLUTION
The mass of the lamina is:
1 2 2
0 0
2 221
00
131 2
00
( , ) (1 3 )
32
84 (1 ) 4
3 3
x
D
y x
y
m x y dA x y dy dx
yy xy dx
xx dx x
P2412.4
Example 2 SOLUTION
Then, Formulas 5 give:
1 2 2 238 0 0
2 221 2
00
12 41 33
2 00
1( , ) ( 3 )
33
8 2
3 3( )
2 2 4 8
x
D
y x
y
x x x y dA x x xy dy dxm
yxy x y x dx
x xx x dx
P2512.4
Example 2 SOLUTION
1 2 2 238 0 0
2 22 2 31
00
1 2 314 0
12 43
0
1( , ) ( 3 )
33
8 2 2 3
(7 9 3 5 )
1 117 9 5
4 2 4 16
x
D
y x
y
y y x y dA y xy y dy dxm
y y yx dx
x x x dx
x xx x
P2612.4
Example 2 SOLUTION
The center of mass is at the point .3 11
, 8 16
P2712.4
Example 3
The density at any point on a semicircular lamina is proportional to the distance from the center of the circle.
Find the center of mass of the lamina.
P2812.4
Example 3 SOLUTION
Let’s place the lamina as the upper half of the circle x2 + y2 = a2. See Figure 6. Then, the distance from a
point (x, y) to the center of the circle (the origin) is:
2 2x y
P2912.4
Example 3 SOLUTION
Therefore, the density function is:
where K is some constant.Both the density function and the shape of the
lamina suggest that we convert to polar coordinates. Then, and the region D is given by:
0 ≤ r ≤ a, 0 ≤ ≤
2 2( , )x y K x y
2 2x y r
P3012.4
Example 3 SOLUTION
Thus, the mass of the lamina is:
2 2
0 0
2
0 0
3 3
0
( , )
( )
3 3
D D
a
a
a
m x y dA K x y dA
Kr r dr d
K d r dr
r K aK
P3112.4
Example 3 SOLUTION
Both the lamina and the density function are symmetric with respect to the y-axis. So, the center of mass must lie on the y-axis, that is,
= 0x
Fig. 16.5.6, p. 1018
P3212.4
Example 3 SOLUTION
The y-coordinate is given by:
3 0 0
33 0 0
4
03
0
4
3
1 3( , ) sin ( )
3sin
3[ cos ]
4
3 2 3
4 2
a
D
a
a
y y x y dA r Kr r dr dm K a
d r dra
r
a
a a
a
P3312.4
Example 3 SOLUTION
Thus, the center of mass is located at the point (0, 3a/(2)).
P3412.4
MOMENT OF INERTIA
The moment of inertia (also called the second moment) of a particle of mass m about an axis is defined to be mr2, where r is the distance from the particle to the axis. We extend this concept to a lamina with density
function (x, y) and occupying a region D by proceeding as we did for ordinary moments.
P3512.4
MOMENT OF INERTIA
Thus, we: Divide D into small rectangles. Approximate the moment of inertia of each
subrectangle about the x-axis. Take the limit of the sum as the number of
subrectangles becomes large.
P3612.4
MOMENT OF INERTIA (X-AXIS)
The result is the moment of inertia of the lamina about the x-axis:
* 2 * *
max , 01 1
2
lim ( ) ( , )
( , )
i j
m n
x ij ij ij ijx y
i j
D
I y x y A
y x y dA
P3712.4
MOMENT OF INERTIA (Y-AXIS)
Similarly, the moment of inertia about the y-axis is:
* 2 * *
max , 01 1
2
lim ( ) ( , )
( , )
i j
m n
y ij ij ij ijx y
i j
D
I x x y A
x x y dA
P3812.4
MOMENT OF INERTIA (ORIGIN)
It is also of interest to consider the moment of inertia about the origin (also called the polar moment of inertia):
Note that I0 = Ix + Iy.
* 2 * 2 * *0 max , 0
1 1
2 2
lim [( ) ( ) ] ( , )
( ) ( , )
i j
m n
ij ij ij ij ijx yi j
D
I x y x y A
x y x y dA
P3912.4
Example 4
Find the moments of inertia Ix , Iy , and I0 of a homogeneous disk D with: Density (x, y) = Center the origin Radius a
P4012.4
Example 4 SOLUTION
The boundary of D is the circle
x2 + y2 = a2
In polar coordinates, D is described by:
0 ≤ ≤ 2, 0 ≤ r ≤ a
P4112.4
Example 4 SOLUTION
Let’s compute I0 first:22 2 2
0 0 0
2 3
0 0
4 4
0
( )
24 2
a
D
a
a
I x y dA r r dr d
d r dr
r a
P4212.4
Example 4 SOLUTION
Instead of computing Ix and Iy directly, we use the facts that Ix + Iy = I0 and Ix = Iy (from the symmetry of the problem).
Thus,4
0
2 4x y
I aI I
P4312.4
MOMENTS OF INERTIA
In Example 4, notice that the mass of the disk is:
m = density × area = (a2)
P4412.4
MOMENTS OF INERTIA
So, the moment of inertia of the disk about the origin (like a wheel about its axle) can be written as:
Thus, if we increase the mass or the radius of the disk, we thereby increase the moment of inertia.
42 2 21 1
0 2 2( )2
aI a a ma
P4512.4
MOMENTS OF INERTIA
In general, the moment of inertia plays much the same role in rotational motion that mass plays in linear motion. The moment of inertia of a wheel is what makes it
difficult to start or stop the rotation of the wheel. This is just as the mass of a car is what makes it
difficult to start or stop the motion of the car.
P4612.4
RADIUS OF GYRATION
The radius of gyration of a lamina about an axis is the number R such that
mR2 = Iwhere: m is the mass of the lamina. I is the moment of inertia about the given axis.
P4712.4
RADIUS OF GYRATION
Equation 9 says that: If the mass of the lamina were concentrated at a
distance R from the axis, then the moment of inertia of this “point mass” would be the same as the moment of inertia of the lamina.
P4812.4
RADIUS OF GYRATION
In particular, the radius of gyration with respect to the x-axis and the radius of gyration
with respect to the y-axis are given by:
yx
2 2x ymy I mx I
P4912.4
RADIUS OF GYRATION
Thus, is the point at which the mass of the lamina can be concentrated without changing the moments of inertia with respect to the coordinate axes. Note the analogy with the center of mass.
( , )x y
P5012.4
Example 5
Find the radius of gyration about the x-axis of the disk in Example 4. As noted, the mass of the disk is m = a2. So, from Equations 10, we have:
So, the radius of gyration about the x-axis is , which is half the radius of the disk.
4 212 4
2 4x aI a
ym a
12y a
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