ading stiffness coefficient</span>
            <span class="keyword">switch</span> this.load_stiff_formula
                <span class="keyword">case</span> this.ENERGY
                    this.stiff = 1.053*(v0*r*y^2*sqrt(m))^(2/5);
                <span class="keyword">case</span> this.OVERLAP
                    this.stiff = 1.053*(v0*r*y^2*sqrt(m))^(2/5) * exp(-atan(beta)/beta)^2;
                <span class="keyword">case</span> this.TIME
                    this.stiff = 1.198*(v0*r*y^2*sqrt(m))^(2/5) 