From the form of the moment of inertia tensor introduced in Eq. (B.24)

it is clear that is

since
is unit length, and projecting it onto any other vector
can only shorten it or leave it unchanged. That is,
, with equality occurring for
for any nonzero
. Zooming out,
*of course* we expect any moment of inertia
for a positive
mass
to be nonnegative. Thus,
is *symmetric
nonnegative definite*. If furthermore
and
are not
collinear, *i.e.*, if there is any nonzero angle between them, then
is *positive definite* (and
). As is well known in
linear algebra [332], real, symmetric, positive-definite
matrices have *orthogonal eigenvectors* and *real, positive
eigenvalues*. In this context, the orthogonal eigenvectors are
called the *principal axes of rotation*. Each corresponding
eigenvalue is the moment of inertia about that principal axis--the
corresponding principal moment of inertia. When angular velocity
vectors
are expressed as a linear combination of the principal
axes, there are no cross-terms in the moment of inertia tensor--no
so-called *products of inertia*.

The three principal axes are *unique* when the eigenvalues of
(principal moments of inertia) are *distinct*. They are
not unique when there are repeated eigenvalues, as in the example
above of a disk rotating about any of its diameters
(§B.4.4). In that example, one principal
axis, the one corresponding to eigenvalue
, was
(*i.e.*,
orthogonal to the disk and passing through its center), while any two
orthogonal diameters in the plane of the disk may be chosen as the
other two principal axes (corresponding to the repeated eigenvalue
).

Symmetry of the rigid body about any axis
(passing through the
origin) means that
is a principal direction. Such a symmetric
body may be constructed, for example, as a *solid of
revolution*.^{B.26}In rotational dynamics, this case is known as the *symmetric top*
[272]. Note that the center of mass will lie
somewhere along an axis of symmetry. The other two principal axes can
be arbitrarily chosen as a mutually orthogonal pair in the (circular)
plane orthogonal to the
axis, intersecting at the
axis. Because of the circular symmetry about
, the two
principal moments of inertia in that plane are equal. Thus the moment
of inertia tensor can be diagonalized to look like

where is the principal moment of inertia about , and is the (twice repeated) principal moment of inertia about the two axes in the circular-symmetry plane. We saw in §B.4.5 (Perpendicular Axis theorem) that if the mass distribution is planar, then .

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