Chapter 8: Applications of Triple Integration
Section 8.4: Moments of Inertia (Second Moments)
Example 8.4.3
If R is the region common to the two cylinders x2+y2=1 and x2+z2=1, and δx,y,z=3+x+y+z is the density in R, obtain the moments of inertia and the radii of gyration about the Cartesian coordinate-axes.
(See Example 8.1.8.)
Solution
Maple Solution - Interactive
Initialize
Context Panel: Assign Name
δ=3+x+y+z→assign
The calculations for the moments of inertia are detailed in Table 8.4.3(a) where the iterated integrals are a modification of the contents of Table 8.1.8(e).
Context Panel: Evaluate and Display Inline
Context Panel: Approximate≻5 (digits)
Ix=∫−11∫−1−x21−x2∫−1−x21−x2δ y2+z2 ⅆz ⅆy ⅆx→assign
Ix = 12815→at 5 digits8.5333
Iy=∫−11∫−1−x21−x2∫−1−x21−x2δ x2+z2 ⅆz ⅆy ⅆx→assign
Iy = 11215→at 5 digits7.4667
Iz=∫−11∫−1−x21−x2∫−1−x21−x2δ x2+y2 ⅆz ⅆy ⅆx→assign
Iz = 11215→at 5 digits7.4667
Table 8.4.3(a) Calculations for the moments of inertia
The total mass m and the radii of gyration are given in Table 8.4.3(b).
m=∫−11∫−1−x21−x2∫−1−x21−x2δ ⅆz ⅆy ⅆx→assign
m = 16
kx=Ix/m→assign
kx = 215⁢30→at 5 digits0.73028
ky=Iy/m→assign
ky = 115⁢105→at 5 digits0.68314
kz=Iz/m→assign
kz = 115⁢105→at 5 digits0.68314
Table 8.4.3(b) Radii of gyration
Maple Solution - Coded
Define the density.
δ≔3+x+y+z:
Obtain the moments of inertia
Qx≔Intδ y2+z2,z=−1−x2..1−x2,y=−1−x2..1−x2,x=−1..1
∫−11∫−−x2+1−x2+1∫−−x2+1−x2+13+x+y+z⁢y2+z2ⅆzⅆyⅆx
Ix≔valueQx
12815
Qy≔Intδ x2+z2,z=−1−x2..1−x2,y=−1−x2..1−x2,x=−1..1
∫−11∫−−x2+1−x2+1∫−−x2+1−x2+13+x+y+z⁢x2+z2ⅆzⅆyⅆx
Iy≔valueQy
11215
Qz≔Intδ x2+y2,z=−1−x2..1−x2,y=−1−x2..1−x2,x=−1..1
∫−11∫−−x2+1−x2+1∫−−x2+1−x2+13+x+y+z⁢x2+y2ⅆzⅆyⅆx
Iz≔valueQz
Obtain the total mass m
M≔Intδ,z=−1−x2..1−x2,y=−1−x2..1−x2,x=−1..1
∫−11∫−−x2+1−x2+1∫−−x2+1−x2+13+x+y+zⅆzⅆyⅆx
m≔valueM
16
Obtain the radii of gyration
kx≔Ix/m
215⁢30
ky≔Iy/m
115⁢105
kz≔Iz/m
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