i am posting this question to make the lives of many people easier in mechanics...[1]
so the concept of reduced mass goes as follows...
there are two types of masses positive mass and negative massrelate to +charge and - charge
the properties of negative is just the opposite to that of positive mass...
for example earth attracts positive mass towards its centre while it repels negative mass from its centre...positive mass experiences upthrust whilst negative mass experiences downthrust!!
so the common POSTULATES of negative mass concept are as follows...
like masses attract each other and unlike masses repel each other...
this is opposite to the concept of electricity and magnetism where likes repel and unlikes attract...
gravitational field moves towards the postive mass and moves away from negative charge
potential due to positive mass is additive whilst that due to negative mass is subtractive....
49now minch had a doubt whch i want to solve using this concept
requesting him to post that soln in here....
3bhiyya can u plz explain in detail about negative and positive mass in relation to this doubt
my doubt ::::here is the figure , a small sphere is cut off frm the earth of diameter R OF EARTH 
now i want to know what force of attraction will the body posess and the deviation between initial and later
49so now we are considering the sphere to be full + a negatve mass present in cut part...so that the mass of cut part becomes zero
mass of cut part is M/8
so we are considering the sphere to be full and presence of a negative mass of -M/8
now the full sphere will creae a positive that is attractive field towards it
and the negative mass will create a negative or repelling field...
49
so the shaded portion contains negative mass
so now let us consider a point in the line joining the centres of the 2 spheres...
49supposing P is x distance away from centre of bigger sphere,,,so it is x-R/2 distance away from smaller sphere,,
49so field at P due to bigger sphere= GM/x2
field due to negative mass=-G(M8)/(x-R/2)2
49so total field E= GMx2-GM8(x-R2)2
and force experienced= mE
put x=2R for ur problem....
attractive force is taken as + and repulsive force as -..