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Sketch 1: Instantaneous center P of a moving plane. The instant center of rotation (also known as instantaneous velocity center, [1] instantaneous center, or pole of planar displacement) of a body undergoing planar movement is a point that has zero velocity at a particular instant of time.
The constants listed here are known values of physical constants expressed in SI units; that is, physical quantities that are generally believed to be universal in nature and thus are independent of the unit system in which they are measured.
Speed is the magnitude of velocity (a vector), which indicates additionally the direction of motion. Speed has the dimensions of distance divided by time. The SI unit of speed is the metre per second (m/s), but the most common unit of speed in everyday usage is the kilometre per hour (km/h) or, in the US and the UK, miles per hour (mph).
In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter (the combined center of mass) or, if one body is much more massive than the other bodies of the system combined, its speed relative to the center of mass of the most massive body.
The speed at which light propagates through transparent materials, such as glass or air, is less than c; similarly, the speed of electromagnetic waves in wire cables is slower than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v ).
The speed of gravitational waves in the general theory of relativity is equal to the speed of light in vacuum, c. [3] Within the theory of special relativity, the constant c is not only about light; instead it is the highest possible speed for any interaction in nature.
In physics and the philosophy of science, instant refers to an infinitesimal interval in time, whose passage is instantaneous.In ordinary speech, an instant has been defined as "a point or very short space of time," a notion deriving from its etymological source, the Latin verb instare, from in-+ stare ('to stand'), meaning 'to stand upon or near.' [1]
Speed is a scalar, being the magnitude of the velocity vector which in relativity is the four-velocity and in three-dimension Euclidean space a three-velocity. Speed is empirically measured as average speed, although current devices in common use can estimate speed over very small intervals and closely approximate instantaneous speed.