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. 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.