where U N + denotes the relative normal velocity after impact. The velocity Signorini condition should be understood together with the previous conditions g ≥ ≥ 0 , λ λ ≥ ≥ 0 , λ λ ⊥ ⊥ g . The acceleration Signorini condition is considered under closed contact ( g = 0 , U N + = 0 ), as: