( ( friction ( forces ( and ( friction machines ) ) ) ) ; )
(ROOT (NP (NP (NN friction) (NNS forces) (CC and) (NN friction) (NNS machines)) (: ;)))
friction forces and friction machines;
(ROOT (S (NP (NN Friction) (NN force)) (VP (VBZ is) (VP (VBN caused) (PP (IN by) (NP (NP (JJ microscopic) (NNS bumps)) (, ,) (VP (VBZ lumps)) (, ,) (CC and) (NP (NP (NNS imperfections)) (VP (VBG colliding))))))) (. .)))