Final answer:
If a box is moving at constant speed with friction acting against it, the net force is zero, meaning the applied force and friction force are equal and opposite, by Newton's first law of motion.
Step-by-step explanation:
If a box is moving at constant speed and there is friction working against the box, the correct answer to this scenario is A) the net force is zero. This is true because, according to Newton's first law of motion, an object moving with constant velocity has no net external force acting on it. Since the box is moving at constant speed, the force of friction must be exactly balanced by the applied force. If a man applies a force of +50 N to move the box at a constant velocity, the friction opposing this movement must be -50 N so that the net force is zero, thereby complying with the state of constant velocity.
This understanding applies to various scenarios, whether it's a man pushing a box or a car moving down the street with constant velocity. When acceleration is not present, this equilibrium of forces ensures that there is no change in velocity. However, if the box were to start accelerating, it would indicate that the applied force had become greater than the friction force, leading to a nonzero net force, as explained in Newton's second law of motion.