Calculus- Early Transcendentals, 2021a

14.5. Triple Integrals 505
Figure 14.12: A tetrahedron.
Pretty much just the way we did for two dimensions we can use triple integration to compute mass,
center of mass, and various average quantities.
Example 14.12: Average Temperature in a Cube
Suppose the temperature at a point is given by T = xyz. Find the average temperature in the cube
with opposite corners at (0,0,0) and (2,2,2).
Solution. In two dimensions we add up the temperature at “each” point and divide by the area; here we
add up the temperatures and divide by the volume, 8:
∫
1 2 ∫ 2 ∫ 2
xyzdzdydx = 1 ∫ 2 ∫ 2 xyz 2
2
8 0 0 0
8 0 0 2 ∣ dydx = 1 ∫ 2 ∫ 2
xydydx
0
4 0 0
= 1 ∫ 2 xy 2
2
4 2 ∣ dx = 1 ∫ 2
xdx= 1 x 2
2
2 2 2 ∣ = 1.
0
0
0
0
♣
Example 14.13: Mass & Center of Mass of Tetrahedron
Suppose the density of an object is given by xz, and the object occupies the tetrahedron with corners
(0,0,0), (0,1,0), (1,1,0),and(0,1,1). Find the mass and center of mass of the object.
Solution. As usual, the mass is the integral of density over the region:
M =
∫ 1 ∫ 1 ∫ y−x
0 x 0
xzdzdydx =
∫ 1 ∫ 1
0 x
x(y − x) 2
dydx = 1 2
2
∫ 1
0
x(1 − x) 3
dx
3