Chapter 8: Applications of Triple Integration
Section 8.4: Moments of Inertia (Second Moments)
Example 8.4.9
If R is the first-octant region that is bounded by the coordinate planes, and the additional planes x=1, x+y+z=2, and δx,y,z=x y2z3 is the density in R, obtain the moments of inertia and the radii of gyration about the Cartesian coordinate-axes.
(See Example 8.1.29.)
Solution
Maple Solution - Interactive
Initialize
Context Panel: Assign Name
δ=x y2z3→assign
The calculations for the moments of inertia are detailed in Table 8.4.12(a) where the iterated integrals are a modification of the contents of Table 8.1.29(b).
Context Panel: Evaluate and Display Inline
Context Panel: Approximate≻5 (digits)
Ix=∫01∫02−x∫02−x−yδ y2+z2 ⅆz ⅆy ⅆx→assign
Ix = 101851975→at 5 digits0.019586
Iy=∫01∫02−x∫02−x−yδ x2+z2 ⅆz ⅆy ⅆx→assign
Iy = 1613103950→at 5 digits0.015517
Iz=∫01∫02−x∫02−x−yδ x2+y2 ⅆz ⅆy ⅆx→assign
Iz = 18417325→at 5 digits0.010620
Table 8.4.12(a) Calculations for the moments of inertia
The total mass m and the radii of gyration are given in Table 8.4.12(b).
m=∫01∫02−x∫02−x−yδ ⅆz ⅆy ⅆx→assign
m = 25115120
kx=Ix/m→assign
kx = 413805⁢14053490→at 5 digits1.0862
ky=Iy/m→assign
ky = 213805⁢44534930→at 5 digits0.96686
kz=Iz/m→assign
kz = 813805⁢1905090→at 5 digits0.79988
Table 8.4.12(b) Radii of gyration
Maple Solution - Coded
Define the density.
δ≔x y2z3:
Obtain the moments of inertia
Qx≔Intδ y2+z2,z=0..2−x−y,y=0..2−x,x=0..1
∫01∫02−x∫02−x−yx⁢y2⁢z3⁢y2+z2ⅆzⅆyⅆx
Ix≔valueQx
101851975
Qy≔Intδ x2+z2,z=0..2−x−y,y=0..2−x,x=0..1
∫01∫02−x∫02−x−yx⁢y2⁢z3⁢x2+z2ⅆzⅆyⅆx
Iy≔valueQy
1613103950
Qz≔Intδ x2+y2,z=0..2−x−y,y=0..2−x,x=0..1
∫01∫02−x∫02−x−yx⁢y2⁢z3⁢x2+y2ⅆzⅆyⅆx
Iz≔valueQz
18417325
Obtain the total mass m
M≔Intδ,z=0..2−x−y,y=0..2−x,x=0..1
∫01∫02−x∫02−x−yx⁢y2⁢z3ⅆzⅆyⅆx
m≔valueM
25115120
Obtain the radii of gyration
kx≔Ix/m
413805⁢14053490
ky≔Iy/m
213805⁢44534930
kz≔Iz/m
813805⁢1905090
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