F21 = Gravitational on man m2 due to m1 . Let r1 and r2 be position vector of mass m1 and m2 respectively as shown in Fig. r21 is position vector of mass m2 w.r.t. mass m2 .
Similarly, F12 = Gravitational force on mass m1 due to mass m2
Obvisouly r12 = –r21; therefore F12 = –F21
