So what is taking place within our test tube as it rotates with some angular velocity? So, let's suppose this is our test tube. The blue particle is some particle given by mass M. And this entire structure, the entire test tube, is rotating in the following direction with a given angular velocity of Omega. Now, the distance from that particle to the axis of rotation is given by R. So this is the radius of the circle that is circumscribed by this rotating particle shown in blue. So, before we begin our derivation, we want to ask ourselves what are all the forces that are acting on that blue particle as it is rotating inside our test tube? So we have three forces acting on the particle.