Two particles A and B of mass 2 m and m respectively attract each other by mutual gravitational force and no other force acts on them. At time t = 0, A was observed to be at rest and B was moving away from A with a speed u. At a later time t it was observed that B was moving towards A with speed u. Assume no collision has taken place by then. Find work done by the gravitational force in the time interval 0 to t
