On exercise 4.6 from Klepper's Book "Intro do mechanics" there is a plane on a landing lane. It has its engine off but is braking with force $F_b$. Also, it is attachted to a sandbag that does friction $f_s$. It asks how far the plane goes.
I found a solution for this exercise that says that momentum is conserved. But I do not understand how is the momentum conserved when there are external forces acting on the plane (the friction force of the sandbag). why is it conserved?
And if there were only the brakes, would the momentum be conserved?