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To the definition of an ovoid: t tangent, s secant line. In projective geometry an ovoid is a sphere like pointset (surface) in a projective space of dimension d ≥ 3. Simple examples in a real projective space are hyperspheres . The essential geometric properties of an ovoid are:
The ellipsoid is a mathematically defined regular surface with specific dimensions. The geoid, on the other hand, coincides with that surface to which the oceans would conform over the entire Earth if free to adjust to the combined effect of the Earth's mass attraction (gravitation) and the centrifugal force of the Earth's rotation.
Many equations in relativistic physics appear simpler when expressed in geometric units, because all occurrences of G and of c drop out. For example, the Schwarzschild radius of a nonrotating uncharged black hole with mass m becomes r = 2m. For this reason, many books and papers on relativistic physics use geometric units.
The undulation of the geoid N is closely related to the disturbing potential T according to Bruns' formula (named after Heinrich Bruns): N = T / γ , {\displaystyle N=T/\gamma \,,} where γ {\displaystyle \gamma } is the force of normal gravity , computed from the normal field potential U {\displaystyle U} .
For the mass attraction effect by itself, the gravitational acceleration at the equator is about 0.18% less than that at the poles due to being located farther from the mass center. When the rotational component is included (as above), the gravity at the equator is about 0.53% less than that at the poles, with gravity at the poles being ...
However, the equations for the angular variation of velocity are algebraically complex and the plane-wave velocities are functions of the propagation angle are. [6] The direction dependent wave speeds for elastic waves through the material can be found by using the Christoffel equation and are given by [ 7 ]
The equation of a tri-axial ellipsoid centred at the origin with semi-axes a, b and c aligned along the coordinate axes is + + = The equation of a spheroid with z as the symmetry axis is given by setting a = b:
It is possible to include both Dirac and Majorana mass terms in the same theory, which (in contrast to the Dirac-mass-only approach) can provide a “natural” explanation for the smallness of the observed neutrino masses, by linking the right-handed neutrinos to yet-unknown physics around the GUT scale [6] (see seesaw mechanism).