Given two bodies, one with mass and the other with mass , they will orbit the barycenter of the two bodies. The equivalent one-body problem, with the position of one body with respect to the other as the unknown, is that of a single body of mass
where the force on this mass is given by the gravitational force between the two bodies. This is just half the harmonic mean of the two masses.
This can be proven easily. Use Newton's second law, the force exerted by body 2 on body 1 is
The force exerted by body 1 on body 2 is
According to Newton's third law, for every action there is an equal and opposite reaction:
The relative acceleration between the two bodies is given by
So we conclude that body 1 moves with respect to the position of body 2 as a body of mass equal to the reduced mass.
The reduced mass is frequently denoted by the Greek letter
Applying the gravitational formula we get that the position of the first body with respect to the second is governed by the same differential equation as the position of a very small body orbiting a body with a mass equal to the sum of the two masses, because
The reduced mass is always less than or equal to the mass of each body.
"Reduced mass" may also refer more generally to an algebraic term of the form
that simplifies an equation of the form
The reduced mass is typically used as a relationship between two system elements in parallel, such as resistors; whether these be in the electrical, thermal, hydraulic, or mechanical domains. This relationship is determined by the physical properties of the elements as well as the continuity equation linking them.