A box which carries sand is moving with a constant velocity u on a smooth horizontal surface. The sand is removed during the motion of the box such that the sand has the same horizontal velocity u. We know that there is no horizontal force exerts on the box. The box will travels in constant velocity u. However, the mass of the box is decreasing. Its momentum is decreasing. So there is a net force acting on the box. BUT there is no net force. What's wrong ???