In physics, the reduced mass is the "effective" inertial mass appearing in the two-body problem of Newtonian mechanics. It is a quantity which allows the two-body problem to be solved as if it were a one-body problem. Note, however, that the mass determining the gravitational force is not reduced. In the computation, one mass can be replaced with the reduced mass, if this is compensated by replacing the other mass with the sum of both masses. The reduced mass is frequently denoted by
μ
{\displaystyle \mu }
(mu), although the standard gravitational parameter is also denoted by
μ
{\displaystyle \mu }
(as are a number of other physical quantities). It has the dimensions of mass, and SI unit kg.