mks units are specifically designed to conveniently describe those motions which occur in everyday life.
Unfortunately, mks units tend to become rather unwieldy when dealing with motions on very
small scales (e.g., the motions of molecules) or very large scales (e.g., the
motion of stars in the Galaxy). In order to help cope with this problem, a set
of standard prefixes has been devised, which allow the mks units of length, mass, and time
to be modified so as to deal more
easily with very small and very large quantities:
these prefixes are specified in Tab. 1. Thus, a kilometer (km) represents m,
a nanometer (nm) represents
m, and a femtosecond (fs) represents s. The standard prefixes
can also be used to modify the units of derived quantities.