Chapter 8: Applications of Triple Integration
Section 8.4: Moments of Inertia (Second Moments)
Example 8.4.7
If R is the first-octant region enclosed by the cylinder x2+z2=4 and the plane y=3, and δx,y,z=2 x2+3 y2+4 z2 is the density in R, obtain the moments of inertia and the radii of gyration about the Cartesian coordinate-axes.
(See Example 8.1.27.)
Solution
Maple Solution - Interactive
Initialize
Context Panel: Assign Name
δ=2 x2+3 y2+4 z2→assign
The calculations for the moments of inertia are detailed in Table 8.4.10(a) where the iterated integrals are a modification of the contents of Table 8.1.27(b).
Context Panel: Evaluate and Display Inline
Context Panel: Approximate≻5 (digits)
Ix=∫02∫04∫04−x2δ y2+z2 ⅆz ⅆy ⅆx→assign
Ix = 1265615⁢π→at 5 digits2650.7
Iy=∫02∫04∫04−x2δ x2+z2 ⅆz ⅆy ⅆx→assign
Iy = 192⁢π→at 5 digits603.19
Iz=∫02∫04∫04−x2δ x2+y2 ⅆz ⅆy ⅆx→assign
Iz = 1249615⁢π→at 5 digits2617.2
Table 8.4.10(a) Calculations for the moments of inertia
The total mass m and the radii of gyration are given in Table 8.4.10(b).
m=∫02∫04∫04−x2δ ⅆz ⅆy ⅆx→assign
m = 88⁢π
kx=Ix/m→assign
kx = 1165⁢261030→at 5 digits3.0964
ky=Iy/m→assign
ky = 211⁢66→at 5 digits1.4771
kz=Iz/m→assign
kz = 115⁢2130→at 5 digits3.0768
Table 8.4.10(b) Radii of gyration
Maple Solution - Coded
Define the density.
δ≔2 x2+3 y2+4 z2:
Obtain the moments of inertia
Qx≔Intδ y2+z2,z=0..4−x2,y=0..4,x=0..2
∫02∫04∫0−x2+42⁢x2+3⁢y2+4⁢z2⁢y2+z2ⅆzⅆyⅆx
Ix≔valueQx
1265615⁢π
Qy≔Intδ x2+z2,z=0..4−x2,y=0..4,x=0..2
∫02∫04∫0−x2+42⁢x2+3⁢y2+4⁢z2⁢x2+z2ⅆzⅆyⅆx
Iy≔valueQy
192⁢π
Qz≔Intδ x2+y2,z=0..4−x2,y=0..4,x=0..2
∫02∫04∫0−x2+42⁢x2+3⁢y2+4⁢z2⁢x2+y2ⅆzⅆyⅆx
Iz≔valueQz
1249615⁢π
Obtain the total mass m
M≔Intδ,z=0..4−x2,y=0..4,x=0..2
∫02∫04∫0−x2+42⁢x2+3⁢y2+4⁢z2ⅆzⅆyⅆx
m≔valueM
88⁢π
Obtain the radii of gyration
kx≔Ix/m
1165⁢261030
ky≔Iy/m
211⁢66
kz≔Iz/m
115⁢2130
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