Consider two identical metallic spheres of radius R each having charge Q and mass m. Their centers have an initial separation of 4R. Both the spheres are given an initial speed of u towards each other. The minimum value of u, so that they can just touch each other is : (Take $k=\frac{1}{4 \pi \epsilon_0}$ and assume $k Q^2>G m^2$ where G is the Gravitational constant)