I'm trying to figure this out. Suppose a box is on a conveyor belt, and the conveyor belt is moving faster and faster to the right, so that static friction has already been overcome. This means that relative to the conveyor belt, the box is accelerating to the left. But what is the net force acting on the box in this case? Where does this force come from? If I draw a free body diagram of the box, I can see there is a kinetic friction force acting to the right, a normal force and a gravitational force. So how can the object be moving towards the left relative to the conveyor belt?