When using this method for unilateral constraints between two rigid bodies, the Signorini condition alone is not enough to model the impact process, so impact laws, which give the information about the states before and after the impact, are also required. For example, when the Newton restitution law is employed, a coefficient of restitution will be defined as: e = − − U N + / U N − − , where U N − − denotes the relative normal velocity before impact.