I don't get it. Two objects (A and B) on a smooth horizontal plane in contact with each other. A force is applied on object A and the objects move to the right together. However, if we were to draw a free body diagram of each object, there would be a force acting like a frictional force on object A but not B. Imagine a free body diagram of each object in that system. P is the force between each object A and B. P acts against object A but in the diagram of object B, P is in the same direction as object B. I understand that this force is not frictional force as the objects are on a smooth plane. What force is it? Is it the same as tension of a string? Thanks.