of energy is required to achieve the same size reduction on systems of larger size. However, in terms of the specific energy, [*i.e.*]{} the energy imparted to the fragmenting system divided by the total mass $E_0/m_{tot}$, all the characteristic quantities show a universal behavior, especially the critical value of the specific energy is independent of the system size [@transition; @proj; @twodisc; @granulate; @poto; @simu]. For impacting discs, however, the critical value of the specific energy shows a clear dependence on the size of disc $R$ as it is illustrated in Fig. \[specific\]. The larger the disc, the higher the energy density is required to break it into pieces. A possible explanation is that for larger discs, a larger part of the imparted energy goes into the motion of the fragments, lowering the efficiency of break-up.

The total number of fragments $N_f$ is also an important measure of the degree of break-up in the impact process. Fig \[nofra\] shows that the number of fragments $N_f$ is uniquely determined by the normal component of the impact velocity $v_n$, [ *i.e.*]{} the curves obtained at different impact angles present a perfect collapse when plotting them as a function of $v_n$. It can be 