Let `upsilon_(r)` be the relative velocity of approach of two bodies at distance `r` apart. The reduced mass of
the system of two particles is, `mu = (m_(1) m_(2))/(m_(1) + m_(2))`.
According to law of conservation of mechincal energy.
Decrease in potential energy = increase in K.E.
`0 - (-(Gm_(1) m_(2))/(r)) = (1)/(2) mu upsilon_(r)^(2)`
or `(Gm_(1) m_(2))/(r) = (1)/(2) ((m_(1) m_(2))/(m_(1) + m_(2))) upsilon_(r)^(2)`
or `upsilon_(r) = sqrt((2G (m_(1) + m_(2)))/(r))`