Why does the frictional force decrease with the increase of angle of inclination? The frictional force is calculated by the normal force * the coefficient of friction. The normal force is mg*cos(angle). When the angle is 0, the normal force is mg. However, as the angle increases, cos(angle) decreases and so mg*cos(angle) decreases. So the normal force decreases as the angle of inclination increases. It follows that the frictional force decreases since the frictional force is the normal force * the coefficient of friction.