Two rollers have a belt that goes between them, like a conveyerbelt. The assembly stands vertical. The lowest of the rolls have a force that acts on its center, and downwards. My book says that this force F can be written as [tex]F=S_1-S_2[/tex] where S are the forces that the belt produces as a reaction to F. I would have guessed [tex]F=S_1+S_2[/tex] since the two S forces would take up half of the force F each. Why is it not so?