Suppose the contact force exerted by a rough horizontal surface on...

Let the contact force exerted by the surface on the body be $F_c$.

The contact force can be resolved into two mutually perpendicular components:
1. Normal reaction $N$ acting vertically upwards.
2. Frictional force $f$ acting horizontally along the surface.

Let $\theta$ be the angle between the contact force and the vertical. From the resolution of vectors, the frictional force is given by:
$f = F_c \sin \theta$

It is given that the magnitude of the contact force $F_c$ is constant and the angle $\theta$ is reduced.

For an acute angle $\theta$, as $\theta$ decreases, the value of $\sin \theta$ also decreases.

Therefore, the frictional force $f$ will decrease.