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90. We use conservation of mechanical energy. The center of mass is at the midpoint of
the cross bar of the
H
and it drops by
L
/2, where
L
is the length of any one of the rods.
The gravitational potential energy decreases by
MgL
/2, where
M
is the mass of the body.
The initial kinetic energy is zero and the final kinetic energy may be written
1
2
2
I
ω
,
where
I
is the rotational inertia of the body and
is its angular velocity when it is vertical.
Thus,
02
2
=−
+
¡
=
MgL
I
MgL I
//
.
1
2
ωω
Since the rods are thin the one along the axis of rotation does not contribute to the

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