I will illustrate my question using a simple example.
Lets imagine the folowing situation: a big surface, a book on this surface and another smaller book located on the first one. There is friction between the books and between the bigger book. We apply a force which is big enough to accelarate the bigger book to a small velocity and then we decrease the force so the book continues to move with a constant velocity.
The smaller book moves with the bigger book having the same constant velocity. Why ? The forces that act on the bigger book are the force we applied, the kinetic friction from the floor, the static friction from the smaller book and the air resistance. The forces that act on the small book are the same static friction from the bigger book but just in opposite direction and the air resistance.
Why they have the same velocity? The forces that act on them are different and their masses are different so the books have different acceleration but in the end they both end up having the same velocity. What causes the upper book to reach the same velocity ?