A box and its contents have a total mass M = 9 kg. A string passes through a hole in the box, and you pull on the string with a constant force F = 77 N (this is in outer space; there are no other forces acting).
(a) Initially the speed of the box was vi = 5 m/s. After the box had moved a distance w = 2.1 m, your hand had moved an additional distance d = 0.4 (a total distance of w + d), because additional string of length d came out of the box (see figure). What is now the speed vf of the box?