the answer would be 1/7 m/s because the initial momentum in the objects is 0, then the final momentum of the objects must be 0. if the 5 kg object has a momentum of 1 (5 * 1/5) then the other object must be -1. The only velocity that will produce that result is -1/7 m/s, but since speed doesnt take into account the direction of an object.