REduced mass concept

in sums werin say 2 mases r attched 2 d two ends of a sppring then we use this reduced mass wich is nthing bt m1*m2/(m1+m2)

http://en.wikipedia.org/wiki/Reduced_mass

thats kewl that we use it...but hwy we use it and under wht conditions???like wen to use reduced in which type of spring sums??

if the mass of the nucleus is not much greater than the particle revolving around it
then we cant use it as then the centre of mass is now = r(m/m+M)

this can be approximated to 0 only when M>>m not when M is comparable to m(as then the centre of mass will not lie in the nucleus itself)