There is a force acting on the side of the swimming pool because of the water pressure. Figure 11 shows the side of the swimming pool. Which arrow shows the direction of the force acting on the side of the swimming pool?

The force exerted by a fluid (like water) on a surface is always perpendicular (at a right angle) to that surface. In the diagram, the side of the swimming pool is a vertical surface. Therefore, the force from the water pressure must act horizontally, at 90 degrees to the side. All three arrows, A, B, and C, show a force acting horizontally and perpendicular to the wall. Any of them correctly shows the direction of the force. (Note: The magnitude of the force increases with depth, so arrow C would represent a larger force than arrow A, but the direction is the same).