Calculations: The theory behind the balancing of a rotating disk is given in the lecture notes. In summary, the force acting on the disk is related to the magnitude and location of the centre of mass of the out of balance disk, Where r is distance from the axis of rotation to the centre of mass. The direction of Fo is radially from the axis toward the centre of mass. For rG G Similarly if the test mass where attached to a massless, rotating disk, the force would be F = mar Directed from the axis to the test mass. FT When the test mass is put on the out of balance disk, the force is the vector sum of the individual forces. F = F F+