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Motion In Straight Line

Question
CBSEENPH11017672

Two masses m1 and m2 are separated by a distance R. What is the gravitational potential at the centre of mass?

Solution
Let the centre of mass be at a distance r1, from m1 and r2 from m2.
Then,
straight r subscript 1 equals fraction numerator straight R space straight m subscript 2 over denominator straight m subscript 1 plus straight m subscript 2 end fraction     and      straight r subscript 2 equals fraction numerator Rm subscript 1 over denominator straight m subscript 1 plus straight m subscript 2 end fraction 
         
Gravitational potential at the centre of mass is, 
straight V equals straight V subscript 1 plus straight V subscript 2 equals negative Gm subscript 1 over straight r subscript 1 minus Gm subscript 2 over straight r subscript 2 
 equals negative straight G open square brackets fraction numerator straight m subscript 1 left parenthesis straight m subscript 1 plus straight m subscript 2 right parenthesis over denominator straight R space straight m subscript 2 end fraction plus fraction numerator straight m subscript 2 left parenthesis straight m subscript 1 plus straight m subscript 2 right parenthesis over denominator straight R space straight m subscript 1 end fraction close square brackets 
 equals negative fraction numerator straight G left parenthesis straight m subscript 1 plus straight m subscript 2 right parenthesis over denominator straight R space end fraction open parentheses straight m subscript 1 over straight m subscript 2 plus straight m subscript 2 over straight m subscript 1 close parentheses 
 equals negative fraction numerator straight G left parenthesis straight m subscript 1 plus straight m subscript 2 right parenthesis left parenthesis straight m subscript 1 squared plus straight m subscript 2 squared right parenthesis over denominator straight R left parenthesis straight m subscript 1 straight m subscript 2 right parenthesis end fraction