I would like to understand what makes the cart below the moving object move. There is no friction between any of the objects. The cart is moving therefore there is force applied to it. The only force can come from the small object. However, the small object has only mg and normal force acting on it, and mg has no component in the cart moving speed, and normal force has component in the opposite direction, so what makes it move?
Conservation of momentum? But it is not a force.