a force of 100N acts on a ball moving on a surface.the force of friction that must act between the surface of the ball and the surface so that the ball keeps on moving with constant velocity over the surface must be

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