Two identical metallic spheres A and B carry charges +Q and −2Q respectively. The force between them is 'F' newton, when they are separated by a distance 'd' in air. The spheres are allowed to touch each other and are moved back to their initial position. The force between them now is:
Two identical copper spheres carrying charges +Q and -9Q separated by a certain distance has attractive force F, if the spheres are allowed to touch each other and moved to distance of separation 'x', so that the force between them becomes 4F/9. Then x is equal to