Dependance of the centripetal force F_c = mv²/r (a), the radially confining force F_r = ma_r (m: grain mass, a_r: radial acceleration) (b), and the radial position r (c) of grain on time. These measured results correspond to the grain outside the ring which follows the epicycloid trajectory as shown by Fig. 2(a) and Fig. 6(b) III. The origin of polar coordinate is set to the center of the glass ring. The capitals A–C correspond to those in Fig. 6(b) III. The magnitudes of F_c and F_r at point B is smaller than that at point C.