The post-impact motion of a projectile impacting a granular medium exhibits three different regimes: penetration, horizontal motion and rebound. In this paper, a statistical criterion is proposed at low impact velocities (v(o)<= 3m/s) to recognize these three different motions since the trajectories of projectiles are easily traced in discrete dynamical simulation. The effects of initial impact conditions and system variables on movement features of the projectile are mainly considered. Then a critical rebounding and penetrating angles are respectively proposed to accurately distinguish three different post-impact motions. Furthermore, a general formula is proposed to scale these two critical angles, which vary linearly with the impact velocity. The fluctuation and the intermittent time of the resistance force on the projectile exhibit different patterns for three post-impact motions, which may be a consequence of acoustic pulses transmitted along contact networks of particles. (C) 2016 Elsevier B.V. All rights reserved.