[en] Magnetization-vector (rvec M) measurements were made on a thin disk of impure Nb rotated relative to a fixed field rvec H at 4.2 K. For all H, rvec M is found to be separable into a non-rotating diamagnetic component rvec M_d plus a penetrating-flux component rvec M_p that turns rigidly with the sample for small rotation angles. For H below H_c1 on a hysteresis loop, rvec M_p diminishes in size but continues to rotate rigidly up to 360 degree. For H between H_c1 and H_c2, rvec M_p rotates up to and then remains at some critical angle, indicating a constant frictional torque between rvec M_p and the sample, which presumably arises from vortex-flux jumps between pinning centers. Moreover, rvec M_d closely equals -rvec H/4π for H above as well as below H_c1. 6 refs., 3 figs