âse-ye farin (#)
Fr.: axe principal
Any imaginary line passing through the center of curvature of a
→ spherical mirror
or → lens and the corresponding geometrical center.
Principal axis is perpendicular
to the mirror or lens at the pole.
Rays of light parallel to the principal axis of a
→ concave mirror
will appear to converge on a point in front of the mirror somewhere
between the mirror's pole and its center of curvature.
Same as → optical axis.
2) Mechanics: One of three mutually perpendicular axes of an
object about which the → moment of inertia
is maximum or minimum.
More specifically, an object has an infinite number of moments of inertia.
If an object
is not symmetrical about all planes through its
→ center of mass, then
there will be one → axis of rotation
about which the moment of inertia
is largest, and there will be one axis of rotation about which the
moment of inertia is smallest. These two axes will always be
perpendicular to each other and are the principal axes of the object.
The third principal axis of an object is the axis perpendicular to these
In general the → angular momentum
(L) of a body spinning about a
point O is not
in the same direction as the axis of rotation, or that of
the → angular velocity
(ω); that is L is not parallel
to ω. For certain bodies, however, there can be
certain axes for which L and ω are parallel.
In that case L = Iω, where I is the
moment of inertia about the axis in question.
In a symmetric rigid body, the axes of symmetry coincide with
the principal axes
of the moment of inertia
→ principal; → axis.