Dear student
When there is a net force on a body, the body will accelerate.
acceleration means change in velocity with respect to time.
When there is no net force on a body, the body may either be rest or move with constant velocity.
or, there will be o change in velocity w.r.t time
so, here eternal force=100N
friction is an opposing force.
so, for constant velocity, Friction =100N in opposite direction so that net force=0 N
Regards