Final answer:
The net force is the vector sum of forces acting on an object. When an object moves at constant velocity, the force exerted and the force of friction is equal in magnitude and opposite in direction to result in zero net force, according to Newton's first law of motion.
Step-by-step explanation:
The question appears to be related to the concept of Net Force, which is a fundamental topic in Physics. In physics, the net force is the vector sum of all the forces acting on an object. When a free-body diagram shows a northward force and a westward force, the net force can be determined using the Pythagorean theorem for two perpendicular forces. When an object is moving at a constant velocity, as in the case of a man pushing a box, and if the man applies a force of +50 N, the force of friction opposing the motion must also be 50 N but in the opposite direction for the net force to be zero, in compliance with Newton's first law of motion.