So, let's suppose this is the x axis. Going this way is the positive direction, and going that way is the negative direction. So we have two negative forces and one positive force. The positive force is the centrifugal force, and that causes our object to move in this direction. Now, let's suppose that the other direction is the negative direction, and we have two forces pointing in that negative direction. We have the force that is created by that fluid that basically pushes on that particle, and that is the bone force that is FB.