Reduced mass is the "effective" inertial mass appearing in the two-body problem of Newtonian mechanics. This is a quantity with the units of mass, 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 by the reduced mass, if this is compensated by replacing the other mass by the sum of both masses. Given two bodies, one with mass <math>m_{1}</math> and the other with mass <math>m_{2}</math>, 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
- <math>m_{red} = \mu = {1 \over {{1 \over m_1} + {1 \over m_2}}} = {{m_1 m_2} \over {m_1 + m_2}}\ ,</math>
where the force on this mass is given by the gravitational force between the two bodies. This is proven easily using Newton's third law
- <math>F_1 = m_1 a_1 = - m_2 a_2 </math>
then the relative acceleration between the two bodies is given by
- <math>a= a_1-a_2 =({1+{m_1 \over m_2}}) a_1 = ({{m_2+m_1}\over{m_1 m_2}}) m_1 a_1 = F_1/ m_{red} </math>
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 <math>\mu</math>. 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
- <math>{{m_1 m_2} \over {m_{red}}}=m_1+m_2</math>.
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
- <math>x_{red} = {1 \over {{1 \over x_1} + {1 \over x_2}}} = {{x_1 x_2} \over {x_1 + x_2}}</math>
that simplifies an equation of the form
- <math>\ {1\over x_{eq}} = \sum_{i=1}^n {1\over x_i} = {1\over x_1} + {1\over x_2} + \cdots+ {1\over x_n}. </math>
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.
