## Fundamentals of Physics Extended (10th Edition)

As $\theta$ increases, the normal force ($F_{N}$) increases.
Normal force is equal and opposite to the force of gravity on a flat plane like this. So when the force acting downwards increases, the force acting upwards (normal force) increases with it. Increasing $\theta$ increases the vertical component of the force, meaning that more force is pulling the box down. When more force is pulling the box down, more (normal) force is pushing it up.