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. And we also have the frictional force that is due to the electrostatic repulsion between the charges found on the fluid particles and this particle that we are examining and that also opposes motion points in the opposite direction. So these two forces oppose motion, and that's why they point in the opposite direction of motion, because our particle is moving in this direction as our test tube rotates. So the first question is, or the first step is, in our derivation, what equation do we want to use?