Abstract: We introduce two rotation vectors to describe the forced oscillations and driving force，respectively， and solve the problem that the principle of measurement for phase difference based on stroboscopic method is not clear. The analytical phase difference is obtained by using rotation vector method. We find that the phase difference always shows a difference between π/2 and the angle of the white line in organic glass turntable is pointing to.

Key words: Bohr resonance, rotation vector, phase difference