COSMOS: Complete Online Solutions Manual Organization System
Chapter 9, Solution 176.
(a) From sample Problem 9.11, we have at the apex A
Ix =
3
ma 2
10
I y = Iz =
3 1 2
m a + h2
5 4
Now observe that symmetry implies I xy = I yz = I zx = 0
Using Equation (9.48), the equation of the ellipsoid of inertia is
then
I x x2 + I y y 2 + I z z 2 = 1
or
3
3 1
3 1
ma 2 x 2 + m a 2 + h 2 y 2 + m a 2 + h 2 z 2 = 1
10
5 4
5 4
For the ellipsoid to be a sphere, the coefficients must be equal.
Therefore,
3
3 1
ma 2 = m a 2 + h 2
10
5 4
or
a
= 2
h
(b) From Sample Problem 9.11, we have
I x′ =
I y′′ =
and at the centroid C
Then
I y′
3
ma 2
10
3 2 1 2
m a + h
20
4
3 2 1 2
h
m a + h + m
= I z′ =
20
4
4
=
1
m 3a 2 + 2h 2
20
(
2
)
Now observe that symmetry implies
I x′y′ = I y′z′ = I z′x′ = 0
From part a it then immediately follows that
3
1
ma 2 =
m 3a 2 + 2h 2
10
20
(
)
or
a
=
h
2
3
COSMOS Chapter 9 Solution 176
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