NEET 2016 Physics and Chemistry Paper 1

Two identical charged spheres suspended from a common point by two mass less strings of lengths l ,are initially at a distance d(d<<l) a part because of their mutual repulsion. The charges begin to leak from both the spheres at a constant rate. As a result, the spheres approach each other with a velocity v. Then v varies as a function of the distance x between the spheres, as :