If the box is moving at a constant velocity, it means that the net force acting on the box is zero. In this case, the force you are applying (40 N) is balanced by an equal and opposite force of friction.
To determine the friction force, we can use Newton's second law of motion:
Net Force = Mass x Acceleration
Since the box is moving at a constant velocity, its acceleration is zero. Therefore, the net force on the box is zero as well.
The force you are applying (40 N) is the only force acting on the box, so the friction force must be equal in magnitude and opposite in direction to balance it.
Therefore, the friction force acting on the box must also be 40 N, but in the opposite direction.
Note that in this scenario, the friction force required to keep the box moving at a constant velocity is equal to the force you are applying.