1) One of the principal lines through the center of a figure or a
solid, especially, the line which divides the figure or solid
symmetrically.
2) A fixed line along which distances are measured or to which
position are referred.

L. axis "axle, pivot," akin to O.E. eax "axis, axle,"
Gk. axon "axle," Skt. aksa- "axle, axis, beam of a balance;"
PIE base *aks- "axis."

Âsé, from Pers. dialects: Qâyeni asak "the shaft connecting
the plough to the yoke," Lori esi "a pillar
(used to put up a tribal tent)," variants hosi, hosin,
Tabari âssen "the foot of a door on which it turns,"
cf. Skt. ISA- "pole or shafts of a carriage or
plough," Av. aêša "the two shafts," Mod.Pers. xiš
"plough(share)," Gk. oiax "handle of rudder, tiller, helm,"
PIE base *ei-, *oi- "pole, thill."

The → moment of inertia of a body about any given axis
is the moment of inertia about a parallel axis through the center of mass, plus the
moment of inertia about the given axis if the mass were located at the center of mass.
same as → Steiner's theorem.

The → moment of inertia of a plane object
(→ lamina) about an axis perpendicular to the plane is equal
to the sum of the moments of inertia about any two perpendicular axes in the plane.
Thus if x and y axes are in the plane,
I_{z} = I_{x} + I_{y}.

1) The practice and practical side of knowledge or skills, as opposed to the
→ theory.
2) Accepted practice or custom.

M.L. praxis "practice, action," from Gk. praxis "practice, action, doing,"
from stem of prassein "to do, to act."

Varzidâr, from varzid past stem of varzidan,
→ practice, + -âr prefix forming nouns of action, such
as kerdâr, raftâr, didâr, goftâr, jostâr, etc.

principal axis

آسهیِ فرین

âse-ye farin (#)

Fr.: axe principal

1) Optics:
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
two axes.
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
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