We present results for a model that describes particle penetration through a medium. The particle motion is subjected to an alternating external field (bias). We employ theoretical considerations and computer simulations. A random walk model is utilized with an alternating bias factor in the probabilities of jumps, which selectively changes the mode of motion from a pure random walk to a regular ballistic, and the entire range in between. At relatively low bias values, a slower diffusion drift accompanies the particle oscillations. Thus, the particle can penetrate deeper into the bulk of the system. On the other hand, if the bias is sufficiently high, the random walk becomes forbidden. Under these conditions, the alternating bias forces particles to undergo a pure ballistic motion, oscillating inside a limited region of space, thus not being able to penetrate deeper into the system. We find that the threshold condition for the transition from random walk to ballistic motion is very sharp. Therefore, it is possible to separate different particles by adjusting bias to overshoot the jump probability of one sort of them and at the same time to be under the jump probability of the other sort of particles. This result could be of importance to explain the high sensibility of penetration through membranes.