COSMOS: Complete Online Solutions Manual Organization System
Chapter 9, Solution 131.
Consider shell to be formed by removing hemisphere of radius r
hemisphere of radius r + t.
For hemisphere:
from
I =
2 2
mr
5
m = ρV =
Area =
1
4π r 2 = 2π r 2
2
(
)
1 4 3 2
ρ π r = ρπ r 3
2 3
3
I =
Thus
2 2
4
3 2
ρπ r 5
ρπ r r =
53
15
For hemispherical shell:
I =
4
4
5
ρπ ( r + t ) − r 5 = ρπ r 5 + 5r 4t + 10r 3t 2 + ... − r 5
15
15
Neglect terms with powers of t > 1,
I =
4
ρπ r 4t
3
Mass of shell:
(
I = 2 ρπ r 2t
(
)
m = ρV = ρ tA = ρ t 2π r 2 = 2 ρπ r 2t
) 23 r
2
=
2 2
mr
3
I =
2 2
mr
3
Radius of gyration:
k2 =
I
2
= r2
m 3
k = 0.816r
COSMOS Chapter 9 Solution 131
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