Chapter 8: Applications of Triple Integration
Section 8.4: Moments of Inertia (Second Moments)
Example 8.4.8
If R is the region bounded by the paraboloid z=4−x2−y2 and the plane z=0, and δr,θ,z=z2r sinθ/3 is the density in R, obtain the moments of inertia and the radii of gyration about the Cartesian coordinate-axes.
(See Example 8.1.28.)
Solution
Maple Solution - Interactive
Initialize
Context Panel: Assign Name
δ=z2r sinθ/3→assign
The calculations for the moments of inertia are detailed in Table 8.4.11(a) where the iterated integrals are a modification of the contents of Table 8.1.28(c).
Context Panel: Evaluate and Display Inline
Context Panel: Approximate≻5 (digits)
Ix=∫02 π∫02∫04−r2δ r2sin2θ+z2 r ⅆz ⅆr ⅆθ→assign
Ix = 40534016175175→at 5 digits231.39
Iy=∫02 π∫02∫04−r2δ r2cos2θ+z2 r ⅆz ⅆr ⅆθ→assign
Iy = 40321024175175→at 5 digits230.18
Iz=∫02 π∫02∫04−r2δ r2 r ⅆz ⅆr ⅆθ→assign
Iz = 16384385→at 5 digits42.556
Table 8.4.11(a) Calculations for the moments of inertia
The total mass m and the radii of gyration are given in Table 8.4.11(b).
m=∫02 π∫02∫04− r2δ r ⅆz ⅆr ⅆθ→assign
m = 4096105
kx=Ix/m→assign
kx = 25005⁢37147110→at 5 digits2.4355
ky=Iy/m→assign
ky = 25005⁢36951915→at 5 digits2.4291
kz=Iz/m→assign
kz = 211⁢33→at 5 digits1.0445
Table 8.4.11(b) Radii of gyration
Maple Solution - Coded
Define the density.
δ≔z2r sinθ/3:
Obtain the moments of inertia
Qx≔Intr δ r2sin2θ+z2,z=0..4−r2,r=0..2,θ=0..2 π
∫02⁢π∫02∫0−r2+4z2⁢r2⁢sin⁡13⁢θ⁢r2⁢sin⁡θ2+z2ⅆzⅆrⅆθ
Ix≔valueQx
40534016175175
Qy≔Intr δ r2cos2θ+z2,z=0..4−r2,r=0..2,θ=0..2 π
∫02⁢π∫02∫0−r2+4z2⁢r2⁢sin⁡13⁢θ⁢r2⁢cos⁡θ2+z2ⅆzⅆrⅆθ
Iy≔valueQy
40321024175175
Qz≔Intr δ r2,z=0..4−r2,r=0..2,θ=0..2 π
∫02⁢π∫02∫0−r2+4z2⁢r4⁢sin⁡13⁢θⅆzⅆrⅆθ
Iz≔valueQz
16384385
Obtain the total mass m
M≔Intr δ,z=0..4−r2,r=0..2,θ=0..2 π
∫02⁢π∫02∫0−r2+4z2⁢r2⁢sin⁡13⁢θⅆzⅆrⅆθ
m≔valueM
4096105
Obtain the radii of gyration
kx≔Ix/m
25005⁢37147110
ky≔Iy/m
25005⁢36951915
kz≔Iz/m
211⁢33
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