If a car is moving on a banked frictionless road one of the component of normal reaction force acts as the centripetal force required for the turn. But normal reaction force is a reaction force. Technically seeing, the the component of normal reaction force should be the result of the centripetal force required for the turn. And turning needs some friction. So how can the road be frictionless? Please explain.
Roads are banked so that we get a component of normal reaction to assist in the turn if friction is not sufficient. For example on very sharp curves on road, in absence of banking, the tires can skid and the results can be deadly. Now if friction is absent, the only force which can help us in turning is the normal reaction. But normal reaction is perpendicular to the road and we want a component of it in the radial direction. So we bank the roads to provide us with the necessary centripetal acceleration.
And normal reaction is a reaction by surface of the road to the weight of the car as well as the centripetal acceleration needed to keep the car in circular motion.