Proper velocity Proper velocity In relativity, proper velocity, also known as celerity, is an alternative to velocity for measuring motion. Whereas velocity relative to an observer is distance per unit time where both distance and time are measured by the observer, proper velocity relative to an observer divides observer-measured distance by the time elapsed on the clocks of the traveling object. Proper velocity equals velocity at low speeds. Proper velocity at high speeds, moreover, retains many of the properties that velocity loses in relativity compared with Newtonian theory. For example, proper velocity equals momentum per unit mass at any speed, and therefore
Moment (physics) of "n" corresponds to a different moment: the 1st moment corresponds to "n" = 1; the 2nd moment to "n" = 2, etc. The 0th moment ("n" = 0) is sometimes called the "monopole moment"; the 1st moment ("n" = 1) is sometimes called the "dipole moment", and the 2nd moment ("n" = 2) is sometimes called the "quadrupole moment", especially in the context of electric charge distributions. Moments of mass: Assuming a density function that is finite and localized to a particular region, outside that region a 1/"r" potential may be expressed as a series of spherical harmonics: The