Two uniform spheres A (Hollow) and B (solid) of same radius R (

Explanation

Force between two charges

When third identical uncharged sphere touched with A and placed at the mid point of AB, then

`F_("net") = F_(1) - F_(2)`
`K ((q^(2)//4))/(r^(2)//4)- K ((q^(2)//2))/(r^(2)//4) = (Kq^(2))/(r^(2))-2.(Kq^(2))/(r^(2))= F- 2F`
`F_("net")= -F` i.e., resultant force is F towards A