A svedberg unit (symbol S, sometimes Sv) is a non-SI unit for sedimentation rate. The sedimentation rate is the rate at which particles of a given size and shape travel to the bottom of the tube under centrifugal force. This reflects the rate at which a molecule sediments under the centrifugal force of a centrifuge. The svedberg is technically a measure of time, and is defined as exactly 10−13 seconds (100 fs).
The unit is named after the Swedish chemist Theodor Svedberg (1884–1971), winner of the 1926 Nobel Prize in chemistry for his work on colloids and his invention of the ultracentrifuge. 50S is the larger subunit of the 70S ribosome of prokaryotes. It is the site of inhibition for antibiotics such as macrolides, chloramphenicol, clindamycin, and the pleuromutilins. It includes the 5S ribosomal RNA and 23S ribosomal RNA.
Factors that affect 
The Svedberg coefficient is a nonlinear function. A particle’s mass, density, and shape will determine its S value. It depends on the frictional forces retarding the particle’s movement, which in turn are related to the average cross-sectional area of the particle.
The sedimentation coefficient is the ratio of the speed of a substance in a centrifuge to its acceleration in comparable units. A substance with a sedimentation coefficient of 26S (26×10−13 s) will travel at 26 microns per second (26×10−6 m/s) under the influence of an acceleration of a million gravities (107 m/s2). Centrifugal acceleration is given as rω2; where r is the radial distance from the rotation axis and ω is the angular velocity in radians per second.
Bigger particles tend to sediment faster and thus have higher svedberg values.
Note that Svedberg units are not directly additive, since they represent a rate of sedimentation, not weight.
By convention, all sedimentation coefficients are expressed in the Svedberg units.
The svedberg is the most important measure used to distinguish ribosomes. Ribosomes are composed of two complex subunits, each of which includes rRNA and protein components. In prokaryotes (bacteria), the subunits are named 30S and 50S for their "size" in Svedberg units. These subunits are made up of three forms of rRNA: 16S, 23S, and 5S.
For bacterial ribosomes, ultracentrifugation yields intact ribosomes (70S) as well as separated ribosomal subunits, the large subunit (50S) and the small subunit (30S). Within cells, ribosomes normally exist as a mixture of joined and separate subunits. The largest particles (whole ribosomes) sediment near the bottom of the tube, whereas the smaller particles (separate 50S and 30S subunits) appear in upper fractions.
See also 
- Svedberg unit - Nobelprize.org