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? Well, we want to use the equation of motion in classical physics, and that is the second law of motion. So we know that the sum of all the forces acting on that particle along the x axis is equal to the mass of that particle multiplied by its acceleration. Now, let's suppose that the velocity of that object, that particle, is constant.