What happens to the object at rest if (i) `f_(s)=0 (ii) f_(s)=f_("ext") (iii) f_(s)="max"`

Explanation

(i) if the object is at rest and so external force is applid on the object the static friction acting on the object is zero `(f_(s)= 0)`
(ii) If the object is at rest and there is an external force applied parallel to the surface then the force of static friction acting on the object is exactly equal to the external force applied on the object `(f_(s) = F_(ex))` But still the static friction `f_(s) ` is less than `mu_(s) N`
(iii) When object begins to side the static friction `(f_(s))` acting on the object attains maximum.