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the internal dynamics of the system

The internal dynamics of the system is given by the acceleration a of the relative position vector r(t).

a = d2r/dt2 = d2r2/dt2 - d2r1/dt2 = a2 - a1 =

= ( F12 + F2,ext )/m2 - ( F21 + F1,ext )/m1

Also using the definition of the reduced mass, we arrive at

μa = F12 + μ( F2,ext /m2 - F1,ext /m1 )

Hence, the acceleration a is determined by the force which mass m1 exerts on mass m2 and the reduced mass μ, except for a residual force which consists of the external forces scaled by the corresponding masses.
Some of the formulas are collected here.





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