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| 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 by the reduced mass, if this is compensated by replacing the other mass by the sum of both masses. The reduced mass is frequently denoted by <math>\scriptstyle \mu </math> ([[Greek alphabet|Greek]] lower case [[Mu (letter)|mu]]), although the [[standard gravitational parameter]] is also denoted by <math>\scriptstyle \mu </math> (and so are [[Mu (letter)#Physics and engineering|a number of other physical quantities]] as well). It has the [[dimensional analysis|dimensions]] of mass, and [[SI unit]] kg.
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| ==Equation==
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| Given two bodies, one with mass ''m''<sub>1</sub> and the other with mass ''m''<sub>2</sub>, 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 <ref>Encyclopaedia of Physics (2nd Edition), R.G. Lerner, G.L. Trigg, VHC publishers, 1991, (Verlagsgesellschaft) 3-527-26954-1, (VHC Inc.) 0-89573-752-3</ref><ref>Dynamics and Relativity, J.R. Forshaw, A.G. Smith, Wiley, 2009, ISBN 978-0-470-01460-8</ref>
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| :<math>m_\text{red} = \mu = \cfrac{1}{\cfrac{1}{m_1}+\cfrac{1}{m_2}} = \cfrac{m_1 m_2}{m_1 + m_2},\!\,</math>
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| where the force on this mass is given by the force between the two bodies.
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| ===Properties===
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| The reduced mass is always less than or equal to the mass of each body:
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| :<math>m_\text{red} \leq m_1, \quad m_\text{red} \leq m_2 \!\,</math>
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| and has the reciprocal additive property:
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| :<math>\frac{1}{m_\text{red}} = \frac{1}{m_1} + \frac{1}{m_2} \,\!</math>
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| which by re-arrangement is equivalent to half of the [[harmonic mean]].
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| ==Derivation==
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| The equation can be derived as follows.
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| ===Newtonian mechanics===
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| {{main|Newtonian mechanics}}
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| Using [[Newton's second law]], the force exerted by body 2 on body 1 is
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| :<math>\bold{F}_{12} = m_1 \bold{a}_1. \!\,</math>
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| The force exerted by body 1 on body 2 is
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| :<math>\bold{F}_{21} = m_2 \bold{a}_2. \!\,</math>
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| According to [[Newton's third law]], the force that body 2 exerts on body 1 is equal and opposite to the force that body 1 exerts on body 2:
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| :<math>\bold{F}_{12} = - \bold{F}_{21}.\!\,</math>
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| Therefore,
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| :<math>m_1 \bold{a}_1 = - m_2 \bold{a}_2. \!\,</math>
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| and
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| :<math>\bold{a}_2=-{m_1 \over m_2} \bold{a}_1. \!\,</math>
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| The relative acceleration '''a'''<sub>rel</sub> between the two bodies is given by
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| :<math>\bold{a}_{\rm rel}= \bold{a}_1-\bold{a}_2 = \left(1+\frac{m_1}{m_2}\right) \bold{a}_1 = \frac{m_2+m_1}{m_1 m_2} m_1 \bold{a}_1 = \frac{\bold{F}_{12}}{m_{\rm red}}. </math>
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| 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.
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| ===Lagrangian mechanics===
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| {{main|Lagrangian mechanics}}
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| Alternatively, a Lagrangian description of the two-body problem gives a [[Lagrangian]] of
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| :<math>L = {1 \over 2} m_1 \mathbf{\dot{r}}_1^2 + {1 \over 2} m_2 \mathbf{\dot{r}}_2^2 - V(| \mathbf{r}_1 - \mathbf{r}_2 | ) \!\,</math>
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| where '''r''' is the position vector of mass ''m<sub>i</sub>'' (of particle ''<math>i</math>''). The potential energy ''V'' is a function as it is only dependent on the absolute distance between the particles. If we define
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| :<math>\mathbf{r} = \mathbf{r}_1 - \mathbf{r}_2 </math> | |
| and let the centre of mass coincide with our origin in this reference frame, i.e.
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| :<math> m_1 \mathbf{r}_1 + m_2 \mathbf{r}_2 = 0 </math>,
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| then
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| :<math> \mathbf{r}_1 = \frac{m_2 \mathbf{r}}{m_1 + m_2} , \mathbf{r}_2 = \frac{-m_1 \mathbf{r}}{m_1 + m_2}.</math>
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| Then substituting above gives a new Lagrangian
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| :<math> L = {1 \over 2}m_\text{red} \mathbf{\dot{r}}^2 - V(r), </math>
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| where
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| :<math>m_\text{red} = \frac{m_1 m_2}{m_1 + m_2} </math>
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| is the reduced mass. Thus we have reduced the two-body problem to that of one body.
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| ==Applications==
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| Reduced mass occurs in a multitude of two-body problems, where classical mechanics is applicable.
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| ===Collisions of particles===
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| In a collision with a [[coefficient of restitution]] ''e'', the change in kinetic energy can be written as
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| :<math>\Delta K = \frac{1}{2}\mu v^2_{\rm rel}(e^2-1)</math>,
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| where v<sub>rel</sub> is the relative velocity of the bodies before [[collision]].
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| For typical applications in nuclear physics, where one particle's mass is much larger than the other the reduced mass can be approximated as the smaller mass of the system. The limit of the reduced mass formula as one mass goes to infinity is the smaller mass, thus this approximation is used to ease calculations, especially when the larger particles exact mass is not known.
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| ===Motions of masses in gravitational fields===
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| In the case of the gravitational potential energy
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| :<math>V(| \mathbf{r}_1 - \mathbf{r}_2 | ) = - \frac{G m_1 m_2}{| \mathbf{r}_1 - \mathbf{r}_2 |} \, ,</math>
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| we find that the position of the first body with respect to the second is governed by the same differential equation as the position of a body with the reduced mass orbiting a body with a mass equal to the sum of the two masses, because
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| :<math>m_1 m_2 = (m_1+m_2) m_\text{red}\!\,</math>
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| ===Non-relativistic quantum mechanics===
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| Consider the [[electron]] (mass ''m<sub>e</sub>'') and [[proton]] (mass ''m<sub>p</sub>'') in the [[hydrogen atom]].<ref>Molecular Quantum Mechanics Parts I and II: An Introduction to QUANTUM CHEMISRTY (Volume 1), P.W. Atkins, Oxford University Press, 1977, ISBN 0-19-855129-0</ref> They orbit each other about a common centre of mass, a two body problem. To analyze the motion of the electron, a one-body problem, the reduced mass replaces the electron mass
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| :<math>m_e \rightarrow \frac{m_em_p}{m_e+m_p} </math>
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| and the proton mass becomes the sum of the two masses
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| :<math>m_p \rightarrow m_e + m_p </math>
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| This idea is used to set up the [[Schrödinger equation]] for the hydrogen atom.
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| ===Other uses===
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| "Reduced mass" may also refer more generally to an [[algebra]]ic term of the form {{Citation needed|date=December 2011}}
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| :<math>x_\text{red} = {1 \over {1 \over x_1} + {1 \over x_2}} = {x_1 x_2 \over x_1 + x_2}\!\,</math>
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| that simplifies an equation of the form
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| :<math>\ {1\over x_\text{eq}} = \sum_{i=1}^n {1\over x_i} = {1\over x_1} + {1\over x_2} + \cdots+ {1\over x_n}.\!\,</math>
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| 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.
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| ==See also==
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| *[[Center-of-momentum frame]]
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| *[[Momentum conservation]]
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| *[[Defining equation (physics)]]
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| *[[Harmonic oscillator]]
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| ==References==
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| {{reflist}}
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| ==External links==
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| *[http://hyperphysics.phy-astr.gsu.edu/hbase/orbv.html#rm ''Reduced Mass'' on HyperPhysics]
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| [[Category:Mechanics]]
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| [[Category:Mass]]
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