Question

In a situation the contact force by a rough horizontal surface on a body placed on it has constant magnitude. If the angle between this force and the vertical is decreased, the frictional force between the surface and the body will
(a) increase
(b) decrease
(c) remain the same
(d) may increase or decrease.

Answer 1

(b) decrease

According to the first law of limiting friction,
f = μN
where f is the frictional force
N is the normal reaction force
μ is the coefficient of static friction

and

N = mg － Fcosθ
where m is the mass of the body
F is the contact force acting on the body

If we decrease the angle between this contact force and the vertical, then Fcosθ increases and the normal reaction force (N) as well as the frictional force (f) decrease.