Axis of rotation

An axis of rotation or rotary axis is a straight line, the rotation (rotation ) of a body or can be carried out.

For bodies that rotate freely in space, eg celestial bodies, the axis of rotation depends in some way on the mass distribution in the body. When changing the distribution of mass in the body, change in general and the rotation axis of the body, but just as that obtained the angular momentum is ( angular momentum conservation ).

Also as the rotation axis is called the straight line at a rotary body to which one can turn it without any change in the body. In this case the axis of rotation is at the same time an axis of symmetry of the rotational body. A distinction is made in this case, a two-fold rotation axes ( Digyren ), threefold rotational axes ( Trigyren ), four-fold rotation axes ( Tetragyren ), fivefold axes of rotation ( Pentagyren ) and sixfold rotation axes ( Hexagyren ). The combination of rotation and reflection leads to the symmetry element of the rotation axes of reflection, that of rotation and inversion of the rotation inversion axes ( Gyroiden ). Are axes of rotation in crystallography designated by their multiplicity, ie Digyren additionally provided with " 2 ", Trigyren with " 3", etc. The Gyroiden be with a slash on the digit. This nomenclature is used in the name of the point groups or crystal classes according to Hermann and Mauguin. For the designation of space groups also play the screw axes combinations of rotation and translation, a role.

Illustration

Make can be graphically the axis of rotation using the wheel. The axis of rotation is perpendicular to the spokes. It is also perpendicular to the wheel tire, which is a circle. In abstract approach to get out without spokes and tires. We will replace the tire by dots that move in a circular path, the diameter of which is irrelevant. The imaginary circle forms a plane on which the rotation axis is vertical.

The rotation axis can now get one direction, that is, they can point in one direction or in opposite directions. When the circle seen from the observer, rotates clockwise, has his eyes in the same direction as the direction of the axis of rotation. This is an agreement (definition). See also the direction of rotation.

Chemistry

The symmetry of a molecule can be described with symmetry elements. This indicates an element of symmetry through which symmetry operation, the molecule can be brought with itself to cover. One of these symmetry elements, the rotation axis Cn ( see figure).