12 12 ,21=12-7i 2 21 12 12 Fig(14-2) The negative sign in Eq(14-3)shows that F points in a direction opposite to ri2, which indicates that the gravitational force is attractiveThe negative sign in Eq(14-3) shows that points in a direction opposite to , which indicates that the gravitational force is attractive. 12 1 2 12 12 12 , r r r r r r     = = −  m1 m1 m1 m2 2 m2 m → F12 → F12 → F12 → F21 → F21  12 r → 12 r → 21 r 21 2 1 21 21 21 , r r r r r r     = = −  Fig(14-2)