Model of the projectile

$$M_p\ddot{\bf z_p}= \sum_{j} \frac{F_{pj}({\bf \vert z_p -z_j\vert})}{{\bf \vert z_p -z_j\vert}}({\bf z_p -z_j})$$ (7)

similar to (4), but the force $F_{pj}$ is found from the equation

$$F_{pj}(z)= \left\{ \begin{array}{lll} 0 & \mbox{if} & z > B... ...\leq B \\ + \infty & \mbox{if} & z \leq A \end{array} \right.$$ (8)

$A = 0.5L, B = 2L$.