The figure above shows the forces exerted on a block that is sliding on a rough horizontal surface: The weight of the block is 500N, the normal force is 500N, the frictional force is 100N, and there is an unknown force F exerted to the right. The acceleration of the block is 0.4 . The value of F is most nearly 20 N 80 N 110 N O 120 N 500 N 100 N F - 500 N Figure not drawn to scale.