Chapter 8: Applications of Triple Integration
Section 8.4: Moments of Inertia (Second Moments)
Example 8.4.4
If R is the region that is inside the cylinder x2+y2=9 and between the planes z=0 and y+z=5, and δr,θ,z=r2+z sinθ/4 is the density in R, obtain the moments of inertia and the radii of gyration about the Cartesian coordinate-axes.
(See Example 8.1.15.)
Solution
Maple Solution - Interactive
Initialize
Context Panel: Assign Name
δ=r2+z sinθ/4→assign
The calculations for the moments of inertia are detailed in Table 8.4.5(a) where the iterated integrals are a modification of the contents of Table 8.1.15(c).
Context Panel: Evaluate and Display Inline
Context Panel: Approximate≻5 (digits)
Ix=∫02 π∫03∫05−r sinθδ r2sin2θ+z2 r ⅆz ⅆr ⅆθ→assign
Ix = 18319367113090→at 5 digits13995.
Iy=∫02 π∫03∫05−r sinθδ r2cos2θ+z2 r ⅆz ⅆr ⅆθ→assign
Iy = 23270475017017→at 5 digits13675.
Iz=∫02 π∫03∫05−r sinθδ r2 r ⅆz ⅆr ⅆθ→assign
Iz = 29108770→at 5 digits4158.4
Table 8.4.5(a) Calculations for the moments of inertia
The total mass m and the radii of gyration are given in Table 8.4.5(b).
m=∫02 π∫03∫05− r sinθδ r ⅆz ⅆr ⅆθ→assign
m = 131322175
kx=Ix/m→assign
kx = 18185738⁢1249646161720165→at 5 digits4.3184
ky=Iy/m→assign
ky = 2553207297⁢82543938310405→at 5 digits4.2689
kz=Iz/m→assign
kz = 343774⁢1179818735→at 5 digits2.3540
Table 8.4.5(b) Radii of gyration
Maple Solution - Coded
Define the density.
δ≔r2+z sinθ/4:
Obtain the moments of inertia
Qx≔Intr δ r2sin2θ+z2,z=0..5−r sinθ,r=0..3,θ=0..2 π
∫02⁢π∫03∫05−r⁢sin⁡θr⁢r2+z⁢sin⁡14⁢θ⁢r2⁢sin⁡θ2+z2ⅆzⅆrⅆθ
Ix≔valueQx
18319367113090
Qy≔Intr δ r2cos2θ+z2,z=0..5−r sinθ,r=0..3,θ=0..2 π
∫02⁢π∫03∫05−r⁢sin⁡θr⁢r2+z⁢sin⁡14⁢θ⁢r2⁢cos⁡θ2+z2ⅆzⅆrⅆθ
Iy≔valueQy
23270475017017
Qz≔Intr δ r2,z=0..5−r sinθ,r=0..3,θ=0..2 π
∫02⁢π∫03∫05−r⁢sin⁡θr3⁢r2+z⁢sin⁡14⁢θⅆzⅆrⅆθ
Iz≔valueQz
29108770
Obtain the total mass m
M≔Intr δ,z=0..5−r sinθ,r=0..3,θ=0..2 π
∫02⁢π∫03∫05−r⁢sin⁡θr⁢r2+z⁢sin⁡14⁢θⅆzⅆrⅆθ
m≔valueM
131322175
Obtain the radii of gyration
kx≔Ix/m
18185738⁢1249646161720165
ky≔Iy/m
2553207297⁢82543938310405
kz≔Iz/m
343774⁢1179818735
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