Final answer:
The net force acting on the box is 200 N to the left, which is an unbalanced force resulting in the box accelerating in that direction.
Step-by-step explanation:
The question you've asked relates to Newton's Second Law of Motion, which is a principle in Physics describing how an object will move when forces are applied to it. The net force acting on the box can be determined by subtracting the smaller force from the larger force, because they are applied in opposite directions. In this case, the big trailer pulls with 400 N to the left, and the little trailer pulls with 200 N to the right, which results in a net force of 400 N - 200 N = 200 N to the left.
This net force is unbalanced because the forces do not cancel each other out; there is a remaining force that will cause the box to accelerate to the left. Net force is a vector quantity that results from summing all forces acting on an object and indicates the total force affecting the object's motion.