If we assume that the wheelbearings have no friction whatsoever then the conveyor travelling backwards at 800 kph,mph or whatever with no thrust from the planes engines as suggested by one of the posters would merely spin the wheels the plane without thrust would remain stationary and not move backwards, apply thrust and the plane would move forwards, the runway conveyor system can increase it's speed if it wishes it makes no difference whatsoever the plane will continue to move forward in relation to it's surroundings to the thrust applied and will take off when the lift is sufficient.